Prepare (t)csh port
tcsh port: work in progress, but still far from working

csh quoting is so inconsistent that I'm now sure that I'm wasting my
time and I'm not even sure I will reach the point where it will be
usable.
(t)csh: fix handling of static prompt

For tcsh, disable the shortcut for a static prompt and handle it always
as a dynamic one.
TODO: optimize the static prompt for tcsh too.
tcsh: almost working!

To do:
- escape \n
- escape ^ and \
Known issue (one more tcsh bug to workaround!): the history of the shell
is polluted with two entries 'angel leave' and 'endif' every time the
prompt is displayed. This is a major usability issue.