spawn: use ~/.cache/public-inbox/inline-c if writable
Despite several memory reductions and pure Perl performance
improvements, Inline::C spawn() still gives us a noticeable
performance boost.
More user-oriented command-line programs are likely coming,
setting PERL_INLINE_DIRECTORY is annoying to users, and so is
is poor performance. So allow users to opt-in to using our
Inline::C code once by creating a `~/.cache/public-inbox/inline-c'
directory.
XDG_CACHE_HOME is respected to override the location of ~/.cache
independent of HOME, according to
https://specifications.freedesktop.org/basedir-spec/0.6/ar01s03.html
v2: use "/nonexistent" if HOME is undefined, since that's
the home of the "nobody" user on both FreeBSD and Debian.