compatability: struct sigaction missing sa_restorer

[Isaac Dunham <idunham@lavabit.com>]

Kernel headers (asm/signal.h) and glibc headers agree that struct
sigaction should have sa_restorer; musl provides __sa_restorer
This is not mandated in X/Open or POSIX, AFAICT.
However, the specification says "struct sigaction includes at least the
following members", so here the glibc/kernel approach is conformant, as
well as being simple and obvious--as opposed to the possible approach:
#ifdef _GNU_SOURCE || _BSD_SOURCE
#define sa_restorer ....
#endif

This prevents building Xvesa; I definitely won't patch it to use
__sa_restorer.  Don't know if sa_restorer is commonly used.

I think that's the last issue, but could be wrong.

As far as __[gu]id_t goes:
$ grep -r  __[gu]id_t /usr/include|wc -l
101

So that's a fair-sized problem (sed already posted, fwiw).