Make PGLC_setlocale() static, and document that it can't be used safely

for any other purpose than PGLC_localeconv()'s internal save/restore of
locale settings.  Fix cash.c to call PGLC_localeconv() rather than
making a direct call to localeconv() --- the old way, if PGLC_localeconv()
had already cached a locale result, it would be overwritten by the first
cash_in or cash_out operation, leading to wrong-locale results later.
Probably no demonstrable bug today, since we only appear to be looking
at the LC_MONETARY results which should be the same anyway, but definitely
a gotcha waiting to strike.