[libc] Remove an unused inclusion

Change-Id: I47b40970bd6a6d04b7c74aff0f9ceda592d24641

[musl][socket] Remove the MSG_NOSIGNAL flag

The MSG_NOSIGNAL flag is intended to suppress delivery of a SIGPIPE
signal on writing into a socket that isn't connected. Since we don't
have signals, this is not needed.

MSG_NOSIGNAL was only very recently added to POSIX, and several
platforms don't have it. As such many systems already have a mechanism
for checking for the existence of this flag and changing build
behavior accordingly.

Change-Id: I84829646d971c3f8b8d8edb933845cabd8d13a10

[musl][pthread] Remove pthread cancellation machinery

Nothing currently in Fuchsia uses this, nor am I aware of any 2nd or
3rd party code that is likely to. I do not believe that, if someone
were to attempt to truly implement pthread_cancel etc. on Magenta,
that the removal of this code and comments would be a meaningful
impediment.

Change-Id: Ib8435a68d0b053ba3f1d0590640ef54c23e59840

[musl] Remove legacy linux syscall mechanism entirely

Change-Id: Ia013bac87e87a7a4528b496ac26da649ad130da0

[musl] Reenable missing-field-initializers for clang

Change-Id: I375e3a15d156a5c8eb1de4af0e380133ec33595d

[musl] Remove linux syscalls and stub some io functions

- Posix message queues are essentially unused, and there is nothing
interesting to keep in the current implementation should we want it.

- wait3 and wait4 are nonstandard.

- chroot is nonstandard.

- We want to just directly call open() and close(). Note that there is
a little bit of careful handling of return values around close(), as
the original syscall wrappers managed the syscall and the conversion
from return-a-negative-error to return-negative-one-and-set-errno in
stages.

Change-Id: If574b115cc3a493d192530866d605bc12a3f5cb8

[musl] Clean up some warnings in musl's make build

Change-Id: Ib305f1d161634156fad8f7303208c49be66c855c

[musl] clang format musl

Change-Id: I3bc6ab7655410514691cd07f08454f3afd41d9b3

[magenta] Initial commit