Header conformance/improvements (part 2)

[Daniel Sabogal <dsabogalcc@gmail.com>]

Here's a list of observations from musl's headers.

tar.h
-----
* TSVTX
this constant is XSI-shaded
glibc exposes it with _XOPEN_SOURCE

unistd.h
--------
* F_LOCK, F_TEST, F_TLOCK, F_ULOCK
these constants are XSI-shaded
glibc exposes them with _XOPEN_SOURCE

stropts.h
---------
* RPROTMASK
this constant is non-standard and not reserved
glibc exposes it with _GNU_SOURCE

signal.h
--------
* int sigqueue(pid_t, int, /* const */ union sigval);
harmless; it just doesn't reflect http://austingroupbugs.net/view.php?id=844

arch/*/bits/termios.h
---------------------
* NLDLY, NL0, NL1
* CRDLY, CR0, CR1, CR2, CR3
* TABDLY, TAB0, TAB1, TAB2, TAB3
* BSDLY, BS0, BS1, FFDLY, FF0, FF1
these constants are XSI-shaded
(so are VTDLY, VT0 and VT1, but the prefix "V" is reserved by posix)
glibc exposes them with _XOPEN_SOURCE

limits.h
--------
* PAGE_SIZE
* NL_LANGMAX
* NZERO
these constants are XSI-shaded
glibc exposes them with _XOPEN_SOURCE (except PAGE_SIZE)

sys/socket.h
------------
* AF_* excluding AF_{INET,INET6,UNIX,UNSPEC}
* MSG_* excluding MSG_{CTRUNC,DONTROUTE,EOR,OOB,NOSIGNAL,PEEK,TRUNC,WAITALL}
* PF_*
* SCM_* excluding SCM_RIGHTS
* SO* excluding SOCK_{DGRAM,RAW,SEQPACKET,STREAM},
SO_{ACCEPTCONN,BROADCAST,DEBUG,DONTROUTE,ERROR,KEEPALIVE,LINGER,OOBINLINE,RCVBUF,RCVLOWAT,RCVTIMEO,REUSEADDR,SNDBUF,SNDLOWAT,SNDTIMEO,TYPE},
SOL_SOCKET, and SOMAXCONN
* CMSG_* excluding CMSG_{DATA,NXTHDR,FIRSTHDR}
these constants/macros are reserved by an XSI-shaded prefix
changing this might be too intrusive; glibc just exposes them

inttypes.h
----------
* wchar_t
this symbol is exposed to the ISO C namespace
AFAICT, this symbol is CX-shaded, and according to n1570 7.8.2.4p1,
it seems to be intended that <stddef.h> be included to expose wchar_t

wchar.h
-------
* FILE
this symbol is exposed to the ISO C namespace
AFAICT, this symbol is CX-shaded, and according to n1570 7.29.2.1p1,
it seems to be intended that <stdio.h> be included to expose FILE