make-fs: Run virt-make-fs in test from builddir, not srcdir.
make-fs: Make check-valgrind do something.
make-fs: Use an anonymous pipe instead of a named pipe.

The previous translation from Perl slavishly followed the Perl code a
bit too much and used a named pipe to communicate between the
uncompressing subprocess and libguestfs tar-in.  From C we can use an
anonymous pipe instead.

This updates commit d3512deb6728dffbb3a958c7168b81dbde56d748.
make-fs: Refactor debug stats code.

Code motion, no functional change.
make-fs: Ensure --partition (on its own) creates an MBR partition.

This fixes commit d3512deb6728dffbb3a958c7168b81dbde56d748.
make-fs: Remove warning from man page about lone --partition argument.

C's getopt_long has no problem with this.