rename RXf_UNUSED flags to match their BASE_SHIFT offset

So they stay stable as I move other flags from extflags to intflags
Move the RXf_ANCH flags to intflags as PREGf_ANCH_xxx and add RXf_IS_ANCHORED as a replacement

The only requirement outside of the regex engine is to identify that there is
an anchor involved at all. So we move the 4 anchor flags to intflags and replace
it with a single aggregate flag RXf_IS_ANCHORED in extflags.

This frees up another 3 bits in extflags.