prompt: disable coloring if no color available or no terminfo support.

This happens on RHEL 5.
dm: only subcommand can be completed

Use file completion otherwise.
prompt: try a "powerline" prompt

This needs a modified powerline font. Dunno how to detect that...
prompt: put cwd basename in bold
prompt: add a check mark in green when everything is OK
prompt: limit size of the prompt to fit in screen
prompt: use heavy check mark when there is no error
prompt: black background for %n@%m

And also ensure we have at least one segment for path
prompt: rework PS2
prompt: keep foreground color for both parts of %n@%m