progreloc: Speed up executable lookup on various platforms. * lib/progreloc.c: Include <errno.h>. (safe_read, full_read): New functions. (find_executable): On GNU/kFreeBSD, FreeBSD, DragonFly, NetBSD, Solaris, prefer the information from the /proc file system to a PATH search.

# Suppress a valgrind message about use of uninitialized memory in strchrnul().
# This use is OK because it provides only a speedup.
{
    strchrnul-value4
    Memcheck:Value4
    fun:strchrnul
}
{
    strchrnul-value8
    Memcheck:Value8
    fun:strchrnul
}