[PATCH] Use execveat syscall in fexecve

[Florian Weimer]

* Andreas Schwab:

> By using execveat we no longer depend on /proc.  The execveat syscall was
> introduced in 3.19, except for a few late comers.

Ideally, this should have a test that executing “/bin/sh -c true”
works, both with an O_PATH descriptor and a regular descriptor.

The implementation itself (including the AT_EMPTY_PATH flag) and the
__ASSUME_EXECVEAT conditions look good to me.