This adds the following new APIs:
 - cap_get_file
 - cap_set_file

'make extra-tests' was a monolithic set of tests that did all sorts of
things: valgrind, tests over local guests, tests with upstream qemu,
tests with upstream libvirt, tests with the appliance attach method.

This made it hard to perform individual tests, eg. just valgrind
testing.  It was also hard to maintain because the tests were not
located in the same directories as the programs and sometimes
duplicated tests that were run elsewhere.

This commit splits up 'make extra-tests' into 5 separate targets:

 make check-valgrind              # run a subset of tests under valgrind
 make check-valgrind-local-guests # test under valgrind with local guests
 make check-with-appliance        # test with attach-method == appliance
 make check-with-upstream-qemu    # test with an alternate/upstream qemu
 make check-with-upstream-libvirt # test with an alternate/upstream libvirt

(You can also still run 'make extra-tests' which is now simply
a rule that runs the above 5 targets in order).

This replaces everything that was in the tests/extra directory,
so that has now gone.

This command was not tested at all.  As a result we didn't notice that
it was broken for a long time (RHBZ#853159).

This adds a test that drives the command through a pty.  It uses the
perl 'Expect' module, although this is not required.

This uses Pod::Simple so it properly parses the input POD and can
generate proper custom output as required specifically by libguestfs.

One immediate benefit is that links between and within manual pages
now work mostly correctly.

Earlier versions of qemu contained a bug in the qcow2 code which
causes qemu to segfault when shutting down and flushing its internal
cache, and this can result in data loss.