include/c: add errexec()

include/c: add errexec()

The new errexec() macro consolidate and unify the way how util-linux
tools react to failed exec()-like functions:

 * exit code 126 when program located, but not usable
 * exit code 127 when could not find program to exec

The exit codes are compatible with coreutils.

Note that all the change is located in c.h; the file exitcodes.h
contains API specific (mkfs, fsck, ...) codes only.

Addresses: https://github.com/karelzak/util-linux/pull/311
Signed-off-by: Karel Zak <kzak@redhat.com>