Encapsulate regexp use in selections.

Encapsulate regexp use in selections.

Add a gmxregex.h header that provides a simple regular expression
matching interface, and use that in selection code instead of using
POSIX regexps directly.  Should make it easier to add regexp support
also on Windows (or at least the complexity is then within just one
file).

Also removed HAVE_SYS_TYPES_H from config.h, as sm_keywords.cpp was the
only place where it was used, and many other files included sys/types.h
unconditionally.

Change-Id: I228ad0cf200bc07a45fd745176add8ee65448789