RFC 2047 requires more characters to be encoded than it is currently done.
Especially, RFC 2047 distinguishes between allowed remaining characters
in encoded words in addresses (From, To, etc.) and other headers, such
as Subject.

Make add_rfc2047() and is_rfc2047_special() location dependent and include
all non-allowed characters to hopefully be RFC 2047 conformant.

This especially fixes a problem, where RFC 822 specials (e. g. ".") were
left unencoded in addresses, which was solved with a non-standard-conforming
workaround in the past (which is going to be removed in a follow-up patch).
Signed-off-by: NJan H. Schönherr <schnhrr@cs.tu-berlin.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>