debian: clean up conditional fuse2fs pacakge build rules

debian: clean up conditional fuse2fs pacakge build rules

There's no point conditionalizing fuse2fs in the control file, since
we control whether or not fuse2fs is built in the rules file, and the
control file is going to be the same when built on all of the debian
build servers.

Also key off of DEB_HOST_ARCH_OS to determine whether or not we are
building for the Hurd, and define it so the right thing happens if
./debian/rules is run by hand on a Hurd system.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>