Rename various regex defined so that they have distinct prefixes based on their usage.

Rename various regex defined so that they have distinct prefixes based on their usage.

RXf_ => flags used in pm_flags argument to regcomp
  and stored in the regex via rx->extflags
PREGf_ => flags stored in rx->intflags
RXapif_ => argument flags for regex named capture api
RX_BUFF_IDX_ => special indexes to represent $` $' $&
  used in the numeric capture buffer api

PREGf is untouched by this change, but RXf_ is split into RXapif and RX_BUFF_IDX_.

p4raw-id: //depot/perl@31497