* release of groffer 0.9.20

* release of groffer 0.9.20

### Split groffer.sh into two files groffer.sh and groffer2.sh.

* groffer.sh:
- Remove the second part of this script.  It is now in
groffer2.sh.
- $_GROFFER2_SH: New variable to point to the installed position
of `groffer2.sh'.  This position is presented using @libdir@.

* groffer2.sh: New script containing the second part of
groffer.sh. This script will be installed in the groffer library
directory @libdir@/groff/groffer, this might be
/usr/local/lib/groff/groffer/groffer2.sh for example.

* Makefile.sub:
- `groffer': Add replacement of @libdir@.
- `install_data': Add the installation of the groffer library
directory and groffer2.sh.
- `uninstall_sub': Delete the installed `groffer2.sh' and the
groffer library directory.

* README_SH:
- Remove the function list.
- Add argument options to the list of used commands.
- Documentation of the splitting of the script.
- Document the possible abbreviation of options.