libc: Add prototypes for various functions we use in __weak_reference().

<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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locales, libconv: Sync with FreeBSD (extensive reach)

What started out as a relatively simply upgrade to libiconv finally
resulted in a simultaneous overhaul to locales, strings, and stdio.
All of t

locales, libconv: Sync with FreeBSD (extensive reach)

What started out as a relatively simply upgrade to libiconv finally
resulted in a simultaneous overhaul to locales, strings, and stdio.
All of these are interdependent and there is no way to upgrade them
individually or in steps.

These cases are similar to what happened with libm where significant
syncing came from NetBSD previously, rendering contributions from
FreeBSD difficult. Libiconv and locales (both ancient) are now in
sync with FreeBSD HEAD.

As several headers were signficantly updated and the mtree was updated
to accommodate the new include/xlocale directory, this commit will
require a full world build. It also may cause some dports to no longer
build due to prototype differences, but the dports will be adjusted.

The regexp library was not being used. It was removed from FreeBSD four
years ago. Since it required collate updates, I took the opportunity to
remove it completely by adding re_comp functionality to 4.3 compat
library like FreeBSD did.

__DragonFly_version has been bumped to 500300.

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libc: Add strchrnul(), a glibc extension

* This function behaves like strchr() but returns a pointer to the
terminating \0 byte of the string instead of NULL if the character
was not found

* It

libc: Add strchrnul(), a glibc extension

* This function behaves like strchr() but returns a pointer to the
terminating \0 byte of the string instead of NULL if the character
was not found

* It was first implemented in glibc-2.1.1

Obtained-from: FreeBSD

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