preliminary FUSE 3 support shim

preliminary FUSE 3 support shim

This shim is imported from another (as-yet-unannounced) AGPL-3+
project I started way back in May 2021.  It turns out the small
C99 + libfuse3 shim is pretty generic aside from its name and
can be easily repurposed for any language or FUSE project.

I'm avoiding existing Perl FUSE bindings since they're
out-of-date w.r.t. FUSE 3 support, and Perl5 has a
thread-unfriendly standard library.

FUSE 3 adds readdirplus which is beneficial for Maildir, and
FUSE FSes are basically unusable without threads.