#436 Extend shellscript edit mode for C shell

[Ben Golding]

The 'shellscript' edit mode seems to know keywords for sh/bash (e.g. 'fi') and highlight these accordingly, but not for csh/tcsh (e.g. 'endif')

Perhaps it would be better to provide distinct modes for Bourne compatible shells (initially could be a rename of existing shellscript mode) and C shell compatible (initially could be a copy/rename of shellscript mode with tweaks such as removing 'fi' and adding 'endif' as a keyword).