EL_RPROMPT support i added to editline(3).
- allow $FTPPROMPT and $FTPRPROMPT to override defaults for the relevant
prompts
- move `%' formatting code from prompt() to expandbuf().
- implement `%.' and `%c', similar to the same % codes in tcsh(1)
(functionality I added to tcsh nearly 6 years ago), except that `%.'
always does `...trailing' and `%c' always does `/<x>trailing'.
- unknown `%foo' codes get printed as `%foo'
ecb8a0755a