sh: Add $'quoting' (C-style escape sequences).

sh: Add $'quoting' (C-style escape sequences).

A string between $' and ' may contain backslash escape sequences similar
to the ones in a C string constant (except that a single-quote must be
escaped and a double-quote need not be). Details are in the sh(1) man
page.

This construct is useful to include unprintable characters, tabs and
newlines in strings; while this can be done with a command substitution
containing a printf command, that needs ugly workarounds if the result
is to end with a newline as command substitution removes all trailing
newlines.

The construct may also be useful in future to describe unprintable
characters without needing to write those characters themselves in 'set
-x', 'export -p' and the like.

The implementation attempts to comply to the proposal for the next issue
of the POSIX specification. Because this construct is not in
POSIX.1-2008, using it in scripts intended to be portable is unwise.

Matching the minimal locale support in the rest of sh, the \u and \U
sequences are currently not useful.

Obtained-from:   FreeBSD SVN rev 221513