lib: util: Add sys_pread_full().

lib: util: Add sys_pread_full().

A pread wrapper that will deal with EINTR and never return a short
read unless pread returns zero meaning EOF.

Thread-safe so may be used as a replacement for pread
inside pread_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 36af33bf9fcdf93fce5ef1520fcb7ddbb07b355e)