meson: add option for fexecve use

meson: add option for fexecve use

There are downsides to using fexecve:

when fexecve is used (for normal executables), /proc/pid/status shows Name: 3,
which means that ps -C foobar doesn't work. pidof works, because it checks
/proc/self/cmdline. /proc/self/exe also shows the correct link, but requires
privileges to read. /proc/self/comm also shows "3".

I think this can be considered a kernel deficiency: when O_CLOEXEC is used, this
"3" is completely meaningless. It could be any number. The kernel should use
argv[0] instead, which at least has *some* meaning.

I think the approach with fexecve/execveat is instersting, so let's provide it
as opt-in.