server: Convert com_touch() to lopsub.

server: Convert com_touch() to lopsub.

Another open-coded option parser bites the dust.

This also gets rid of struct com_touch_options, along with the ugly
convention of passing -1 to the action handler to indicate that the
option was not given.

The documentation was enhanced slightly and now explains in the
[description] section the general concept of the touch command.