imap-send.c: set CURLOPT_USE_SSL to CURLUSESSL_TRY

imap-send.c: set CURLOPT_USE_SSL to CURLUSESSL_TRY

According to the cURL documentation for the CURLOPT_USE_SSL option,
it is only used with plain text protocols that get upgraded to SSL
using the STARTTLS command.

The server.use_ssl variable is only set when we are using a protocol
that is already SSL/TLS (i.e. imaps), so setting CURLOPT_USE_SSL
when the server.use_ssl variable is set has no effect whatsoever.

Instead, set CURLOPT_USE_SSL to CURLUSESSL_TRY when the server.use_ssl
variable is NOT set so that cURL will attempt to upgrade the plain
text connection to SSL/TLS using STARTTLS in that case.

This much more closely matches the behavior of the non-cURL code path.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>