[Tcsh] "Readable" Unicode in setenv

[Kimmo Suominen]

Hi all,

I've committed code that implements the dollar-single-quote feature.
https://github.com/tcsh-org/tcsh/commit/bda86ab754b56d5a8983b554cb1dfffad2f3b43d

What should we do about these differences:

We do:
- \uNNNNNN for unicode code points (six hex digits)
- \xNN and \x{NNNN} for "ASCII" char in hex (how is NNNN ASCII?)

FreeBSD sh(1) documents these:
- \uNNNN and \uNNNNNNNN for unicode code points (four and eight hex digits)
- \xNN for a byte in hex, where NN can be 1 or more hex digits and
only last two are used

We are also missing \cC for a control character. I would have added
this but ran out of steam for now.

Cheers,
+ Kimmo