Add support for "standalone months" to tztime's strftime().

The idea here is that some languages need a different form of the month
name in constructions like "January 2" than in "January 2009", since the
one in the "January 2" case really means "of January."  So with this change,
a format string of "%-B" will use the standalone month, while "%B" will
continue to use the format month.