Add libc wrappers for the new sendmmsg and recvmmsg system calls.
Feedback tb@, miod@, jca@
OK jca@

Use a Thread Information Block in both single and multi-threaded programs.
This stores errno, the cancelation flags, and related bits for each thread
and is allocated by ld.so or libc.a. This is an

Use a Thread Information Block in both single and multi-threaded programs.
This stores errno, the cancelation flags, and related bits for each thread
and is allocated by ld.so or libc.a. This is an ABI break from 5.9-stable!

Make libpthread dlopen'able by moving the cancelation wrappers into libc
and doing locking and fork/errno handling via callbacks that libpthread
registers when it first initializes. 'errno' *must* be declared via
<errno.h> now!

Clean up libpthread's symbol exports like libc.

On powerpc, offset the TIB/TCB/TLS data from the register per the ELF spec.

Testing by various, particularly sthen@ and patrick@
ok kettenis@

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recv() and send() aren't overriden by libpthread (vs recvfrom() and sendto()!)
so wrap them to make internal calls go direct

getpeereid() and sockatmark() are neither used in libc nor in ISO C, so mark
them deprecated and weak

Add PROTO_WRAP() for (almost) all the syscalls that libpthread doesn't
override so that internal calls go direct

ok deraadt@