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.

src/shared/exec-util.c

@@ -447,6 +447,7 @@ ExecCommandFlags exec_command_flags_from_string(const char *s) {
 }
 
 int fexecve_or_execve(int executable_fd, const char *executable, char *const argv[], char *const envp[]) {
+#if ENABLE_FEXECVE
         execveat(executable_fd, "", argv, envp, AT_EMPTY_PATH);
 
         if (IN_SET(errno, ENOSYS, ENOENT) || ERRNO_IS_PRIVILEGE(errno))
@@ -463,6 +464,7 @@ int fexecve_or_execve(int executable_fd, const char *executable, char *const arg
                  * least in case of bash) the script name, $0, will be shown as /dev/fd/nnn, which breaks
                  * scripts which make use of $0. Thus, let's fall back to execve() in this case.
                  */
+#endif
                 execve(executable, argv, envp);
         return -errno;
 }