imap-send: use curl by default when possible

imap-send: use curl by default when possible

Set curl as the runtime default when it is available.
When linked against older curl versions (< 7_34_0) or without curl,
use the legacy imap implementation.

The goal is to validate feature parity between the legacy and
the curl implementation, deprecate the legacy implementation
later on and in the long term, hopefully drop it altogether.

Signed-off-by: Nicolas Morey-Chaisemartin <nicolas@morey-chaisemartin.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>