libc - Implement sigblockall() and sigunblockall()

* Signal safety is becoming a defacto requirement for most of libc and
pthreads. In particular, the memory allocator. Given the chances of
te

libc - Implement sigblockall() and sigunblockall()

* Signal safety is becoming a defacto requirement for most of libc and
pthreads. In particular, the memory allocator. Given the chances of
teaching tens of thousands of programmers about signal safety, and
just making it work in libc and pthreads, only one of these two
possibilities is actually realizable.

In particular, high-level languages have become so complex, and some
applications (chrome, firefox, etc) have become so complex, that the
code is regularly tripping over signal safety issues.

However, implementing signal safety with current mechanisms is extremely
expensive due to the need for multiple system calls. To whit,
DragonFlyBSD now has a mechanism that does not require system calls
in the critical path.

* Implement sigblockall() and sigunblockall(). These functions
leverage the new /dev/lpmap per-thread shared page mechanism
to provide a way to temporary block the dispatch of all maskable
signals without having to make any system calls.

These are extremely fast routines.

- Reentrant / Recursable

- Temporarily blocks any dispatch of a maskable asynchronous signal
to the calling thread. Other threads are not affected... this is
a per-thread mechanism.

- The last sigunblockall() will immediately dispatch any blocked
signals.

- The normal signal mask is not affected by these routines.

- Does not block signals caused by synchronous traps.

- The current recursion count is retained on [v]fork() to ease
coding and to also allow signals to be temporarily blocked across a
fork until the child process is ready to deal with them, if desired.

* Implement signal safety for most of pthreads. All temporary internal
mutexes are now wrapped with sigblockall() and sigunblockall().

* Implement signal safety for the malloc subsystem. All functions
are wrawpped with sigblockall() and sigunblockall().

These implementations make lang/mono and lang/rust far more reliable
than they were before. Where 9 out of 10 builds used to fail, now
they succeed.

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<sys/cdefs.h>: Rework __weak_reference() macro.

My LTO build blew away weak symbols from slim LTO objects.
Use __strong_reference() + weak attribute to allow the compiler to catch
extern declaration

<sys/cdefs.h>: Rework __weak_reference() macro.

My LTO build blew away weak symbols from slim LTO objects.
Use __strong_reference() + weak attribute to allow the compiler to catch
extern declarations and not to fold weak symbols as local ones.

Keep previous version as __weak_reference_asm() in _pthread_stubs.c for
now due to several issues (there is a need to do it in a cleaner way).

lib/libc/gen/ucontext.c:
add missing __DECONST for ucp, shouldn't sigreturn take const ucontext_t?

lib/libc/inet/inet_ntoa.c:
add missing #undef inet_ntoa_r

No symbol changes in libc and librt on normal compilation.

While there, add __weak_symbol attribute for future additions.

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Fix warning.

Remove _THREAD_SAFE depenendancies. Create weakly associated stubs for
posix threading calls used by libc and weak associations for procedures that
a threading library would otherwise have to overri

Remove _THREAD_SAFE depenendancies. Create weakly associated stubs for
posix threading calls used by libc and weak associations for procedures that
a threading library would otherwise have to override. This allows a program
to be linked with libc, or linked with libc + a threading library. libc_r
will be deprecated over time.

Traditionally libc_r had to be linked against a threading library rather then
libc. Linux, FreeBSD, and other UNIXes have moved either t oa thread-safe
libc or a thread-capable libc. The threading capability is accomplished by
making the appropriate thread locking calls and providing weakly referenced
dummy stub functions for the case where a threading library is not linked in.
Certain third party software, namely XFree/XOrg, also assume threading
stubs in libc.

This is precursor work to a port of David Xu's 1:1 threading library and is
also intended as a basis to allow DragonFly to support multiple threading
libraries.

Ported-by: David Rhodus <sdrhodus@gmail.com>
Primarily-ported-from: FreeBSD
Testing and bug fixes by David Xu and Matt Dillon

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