Rename enum labels of PG_Locale_Strategy

Rename enum labels of PG_Locale_Strategy

PG_REGEX_BUILTIN was added in f69319f2f1fb but it did not follow the
same pattern as the previous labels, i.e. PG_LOCALE_*.  In addition to
this, the two libc strategies did not include in the name that they were
related to this library.

The enum labels are renamed as PG_STRATEGY_type[_subtype] to make the
code clearer, in accordance to the library and the functions they rely
on.

Author: Andreas Karlsson
Discussion: https://postgr.es/m/6f81200f-68fd-411e-97a1-d1f291d2e222@proxel.se