[GRASS5] g.manual now with optional MAN support

[Markus Neteler]

Hi,

I have added MAN (man) support to g.manual/5.7, there is
now a new -m flag:

#manual page index as MAN output (instead of browser):
g.manual -im

#manual page index of GRASS/SQL page
g.manual -m sql

#manual page index of module:
g.manual -m d.vect

This new flag permits to read the manual pages in
text mode as well.

If you omit the -m flag, it will launch a web browser
instead as before.

Cheers

 Markus