lib: util: Add sys_pwrite_full().

lib: util: Add sys_pwrite_full().

A pwrite wrapper that will deal with EINTR and never return a short
write unless the file system returns an error. Copes with the
unspecified edge condition of pwrite returning zero by changing
the return to -1, errno = ENOSPC.

Thread-safe so may be used as a replacement for pwrite
inside pwrite_do() thread functions.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14361

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Jeremy Allison <jra@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from commit 3ba7a89cea85d134eacf1e624e011fe6f66146fc)