Add ability to set custom fusermount path

[TingPing]

This is only exposed in Meson as Autotools isn't recommended.

This also removes the 'fuse' option as it seems redundant.

Fixes #486

[smcv]

This is only exposed in Meson as Autotools isn't recommended.

Actually the PR as proposed here would let Autotools users do something like ./configure FUSERMOUNT=fusermount3, which seems sufficient to deal with distros where this is necessary.

(In Debian and maybe others, after replacing FUSE 2 with FUSE 3, fusermount is a symlink to fusermount3 anyway - but not every OS vendor provides that compatibility symlink.)