imap-send: gracefully fail if CRAM-MD5 authentication is requested without OpenSSL

imap-send: gracefully fail if CRAM-MD5 authentication is requested without OpenSSL

Unlike PLAIN, XOAUTH2 and OAUTHBEARER, CRAM-MD5 authentication is not
supported by libcurl and requires OpenSSL. If the user tries to use
CRAM-MD5 authentication without OpenSSL, the previous behaviour was to
attempt to authenticate and fail with a die(error). Handle this in a
better way by first checking if OpenSSL is available and then attempting
to authenticate. If OpenSSL is not available, print an error message and
exit gracefully.

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>