(posix_pedantic): Remove; no longer needed since

(posix_pedantic): Remove; no longer needed since
we need to test this in just one place now.
(usage): Mention -C.
(unquote): Note that \055, \n, etc are escaped.
Do not worry about POSIXLY_CORRECT when warning about ambiguous
escape sequences.
\ at end of string stands for itself.
Do not diagnose invalid backslash escapes: POSIX says the behavior
is unspecified in this case, so we don't need to diagnose it.
(main): Add support for -C (currently an alias for -c).
Do not diagnose 'tr [:upper:] [:upper:], as POSIX does not require
a diagnostic here.