Minor refactor: Use more consistent names for the labels of PG_Locale_Strategy

Hi,

When working on the regex code I noticed that the labels of 
PG_Locale_Strategy had become inconsistent with the addition of 
PG_REGEX_BUILTIN but while at it I also noticed that 
PG_REGEX_LOCALE_WIDE_L and PG_REGEX_LOCALE_1BYTE_L did not make it
obvious that they were libc-related so I propose a new naming scheme:

PG_STRATEGY_<type>[_<subtype>]

I am open for other suggestions of course like keeping the PG_LOCALE_* 
prefix, but in any case I think we should make the enum labels consistent.

Andreas