Various changes to tests and Makefile.

Various changes to tests and Makefile.

Merged all loopback scripts into a single variant; now threads are
enabled by passing --use-threads to loopback.pl instead of having a
separate script variant, and use of Filesys::Statvfs instead of
bogus filler statfs() data is enabled with --use-real-statfs. Also,
loopback now fork()s itself away into a daemon, rather than depending
on external infrastructure to do it; --pidfile can be passed to create
a PID file.

Altered test/helper.pm and test/s/mount.t to use new loopback.pl semantics.

Altered test/statfs.t to optionally use Filesys::Statvfs's statvfs()
instead of raw syscalls and pack masks, if the option is available
to us. If not available, it will try to use syscalls instead.

Cleanups to Makefile to put different options where they belong,
rather than e.g., abusing the 'OBJECT' list to specify libraries
and such. Works with all platforms.

Updates to README and MANIFEST.