gdbserver: cleanup monitor_show_help

After this commit:

  commit 0b04e523
  Date:   Sun Jan 19 14:33:37 2014 -0700

      link gdbserver against libiberty

we can cleanup how the help text is generated in monitor_show_help.
This doesn't change the output that the user will see -- it just folds
multiple monitor_output calls into one.

There should be no user visible change after this commit.