Add the new -r option to grotty. It is similar to the -i option

Add the new -r option to grotty.  It is similar to the -i option
except it tells grotty(1) to use the "reverse video" attribute to
render italic fonts.

* src/devices/grotty/tty.cc (reverse_flag): New global variable.
(SGR_REVERSE, SGR_NO_REVERSE): New macros.
(tty_printer::make_underline, tty_printer::put_color,
tty_printer::end_page): Use it.
(main): Add -r switch.
(usage): Updated.
* src/devices/grotty/grotty.man: Document it.