imap-send: use Apple's Security framework for base64 encoding

Use Apple's supported functions for base64 encoding instead
of the deprecated OpenSSL functions.

Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>