Sometimes users would get confused because an option they were looking
for was not mentioned on a subcommand's man page, and they had not
noticed that the main git-annex man page had a list of common options.
This change lets each subcommand mention the common options, similarly
to how the matching options are handled.
This commit was sponsored by Svenne Krap on Patreon.
* map: Run xdot if it's available in PATH. On OSX, the dot command
does not support graphical display, while xdot does.
* Debian: xdot is a better interactive viewer than dot, so Suggest
xdot, rather than graphviz.
