Make initdb behave sanely when the selected locale has codeset "US-ASCII".

Per discussion, this should result in defaulting to SQL_ASCII encoding.
The original coding could not support that because it conflated selection
of SQL_ASCII encoding with not being able to determine the encoding.
Adjust pg_get_encoding_from_locale()'s API to distinguish these cases,
and fix callers appropriately.  Only initdb actually changes behavior,
since the other callers were perfectly content to consider these cases
equivalent.

Per bug #5178 from Boh Yap.  Not going to bother back-patching, since
no one has complained before and there's an easy workaround (namely,
specify the encoding you want).