Add GROUP_BY_* constants and update WITH_OFFSET.
Add GROUP_BY_* constants and update WITH_OFFSET.

In this way, we have aliases for all PCRE constants used by split() and
match(). WITH_OFFSET works for both.

Metalaka/Central,vcgato29/Central,vcgato29/Central,Metalaka/Central,Grummfy/Central,Grummfy/Central