Add a new section called "EXTENDING LIBGUESTFS" to the
guestfs manual page which contains all the information
previously in "HACKING".

This should allow us to perform filesystem-based write
tests much more quickly, because we don't need to recreate
the filesystem from scratch each time.

This fixes commit 18374b5b7d3154e0b8b8a07e3590f6eee762b58e.

I have diffed the output from the original virt-df with this
new version, and they agree very closely.  Some differences:

 - Old virt-df have a divide-by-zero error in cases where the
   number of used inodes was 0.  New virt-df fixes this.

 - New virt-df uses gnulib human_readable library which displays
   numbers to 3 significant figures for -h output (old version
   used an ad hoc function).

This tool replaces virt-list-filesystems and virt-list-partitions with
a new tool written in C with a more uniform command line structure
and output.

This existing Perl tools are deprecated but remain indefinitely.

With changes in the core API since 1.5, virt-cat was little
more than a Perl wrapper which did some command line argument
processing.  Thus it could easily be rewritten in C.

This version also shares core command line argument processing
with guestfish and guestmount, so the options have changed
slightly (old-style command line *is* supported).

 virt-cat -a disk.img file [file ...]
 virt-cat -d domname file [file ...]

Several other guestfish options are supported including encryption,
and with the new style multiple files can be downloaded.  See the
man page for details.