subcommand: Implement roff and html output for com_man().

subcommand: Implement roff and html output for com_man().

This implements the -m option of the internal man subcommand to specify
the output format for the manual page (text, roff, or html).  html is
generated by running the html postprocessor of groff. Another option,
-b, is introduced to let the user specify the browser for html output.

The commit introduces new public variable $gsu_package whose value is
shown at the bottom left of the man page.

The documentation is updated accordingly, and the example subcommand
of README.md has been enhaced to illustrate how subcommand options
should be formatted to look nice in both html and roff output.