Deprecate the use of regexp.h

<regexp.h> (not to be confused with <regex.h>) is an obsolete and
frankly horrible regular expression-matching API.  It was part of SVID
but was withdrawn in Issue 5 (for reference, we're on Issue 7 now).
It doesn't do anything you can't do with <regex.h>, and using it
involves defining a bunch of macros before including the header.
Moreover, the code in regexp.h that uses those macros has been buggy
since its creation (in 1996) and no one has noticed, which indicates
to me that there are no users.  (Specifically, RETURN() is used in a
whole bunch of cases where it should have been ERROR().)

The header is given a warning and marked deprecated for 2.22.

See:
https://sourceware.org/ml/libc-alpha/2015-07/msg00862.html and
https://sourceware.org/ml/libc-alpha/2015-07/msg00871.html.