Tighten up make_libc_collator() and make_icu_collator().

Tighten up make_libc_collator() and make_icu_collator().

Ensure that error paths within these functions do not leak a collator,
and return the result rather than using an out parameter. (Error paths
in the caller may still result in a leaked collator, which will be
addressed separately.)

In make_libc_collator(), if the first newlocale() succeeds and the
second one fails, close the first locale_t object.

The function make_icu_collator() doesn't have any external callers, so
change it to be static.

Discussion: https://postgr.es/m/54d20e812bd6c3e44c10eddcd757ec494ebf1803.camel@j-davis.com