When -reincarnate-reexec is given we run execv to restart FS.  If
argv[0] isn't a full pathname then execv is going to fail.  While not
common for a FS system started by init, this is a common occurrence
when FS is started from the shell.

Now if execv fails, we'll try execvp.  If that fails too then we'll
fall back on the normal reincarnation behavior.

Previously what would happen in that case is god would descend from
the heavens and become mortal.  Leaving heaven absent, all hope for
reincarnation was lost.

(That is, we'd simply return from reincarnate_protect and the
supervisor process would become the new instance of FS, so the trick
would only work once.)