send-email: be explicit with SSL certificate verification

send-email: be explicit with SSL certificate verification

When initiating an SSL connection without explicitly specifying the
SSL certificate verification mode, Net::SMTP::SSL defaults to no
verification, but recent versions of the module gives a warning
against this use of the default.

Enable certificate verification by default, using /etc/ssl/certs as
the default path for certificates of certificate authorities.  This
path can be overriden by the --smtp-ssl-cert-path command line
option and the sendemail.smtpSSLCertPath configuration variable.

Passing an empty string as the path for CA certificates path disables
the SSL certificate verification explicitly, which does not trigger
the warning from recent versions of Net::SMTP::SSL.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Helped-by: Brian M. Carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>