[pass] [PATCH 1/2] check for pwgen's existance before using

[Jason A. Donenfeld]

If you didn't notice before, I'm trying to keep runtime detection of
things to an absolute minimum. That's why I didn't like the original
darwin ramdisk hacks. Conceding for the statically sourced platform
specific section avoided runtime detection. Still though, I don't want
to clutter this either. Pwgen is always pwgen, on all systems I've
seen, and is tiny and only requires libc. If you have a platform.sh
file to add to the platforms/ directory that requires a different
pwgen name, then I'd consider that. But until then, no.