Enable S/MIME signing of e-mails

Use existing PHPMailer functionality to allow MantisBT to sign outgoing
e-mail notifications.

Fixes 0025764

Signed-off-by: Damien Regad dregad@mantisbt.org

TomekAP's original contribution was modified as follows:

• Fix inconsistent naming of 'g_email_signing_sign_extracerts_file' in
  config_defaults_inc.php, referenced everywhere else as
  'g_email_signing_extracerts_file'
• Rename configs with prefix 'smime' instead of 'signing', to avoid any
  confusion with DKIM which is also about signing
• Use suffix '_file' instead of '_filename'
• Fix whitespace