Don't choke during startup if the environment offers an invalid value

Don't choke during startup if the environment offers an invalid value
for LC_MESSAGES; instead, just press forward, leaving the effective setting
at 'C'.  There is not any very good reason to complain when we are going
to replace the value soon with whatever postgresql.conf says.  This change
should solve the occasionally-reported problem of initdb failing with
'failed to initialize lc_messages'; the current theory is that that is
a reflection of either wrong LANG/LC_MESSAGES or completely broken locale
support.