send-email: set SSL options through IO::Socket::SSL::set_client_defaults

send-email: set SSL options through IO::Socket::SSL::set_client_defaults

When --smtp-encryption=ssl, we use a Net::SMTP::SSL connection,
passing its ->new all the options that would otherwise go to
Net::SMTP->new (most options) and IO::Socket::SSL->start_SSL (for the
SSL options).

However, while Net::SMTP::SSL replaces the underlying socket class
with an SSL socket, it does nothing to allow passing options to that
socket.  So the SSL-relevant options are lost.

Fortunately there is an escape hatch: we can directly set the options
with IO::Socket::SSL::set_client_defaults.  They will then persist
within the IO::Socket::SSL module.

Signed-off-by: Thomas Rast <tr@thomasrast.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>