[MERGE] audit anchoring in re_intuit_start()

[MERGE] audit anchoring in re_intuit_start()

This series of commits audits all the code in re_intuit_start()
that handles anchoring, e.g. /^.../, /^.../m, /\G.../.

It fixes a couple of potential performance issues, and makes the code a
lot safer with with \G. (Until recently, intuit was skipped altogether
under \G, so its \G handling isn't well developed and tested.)