send all the mails only by phpMailer,
remove obsolete phpMailer config options,
minor send mail optimization - used kept alive SMTP connection,
send signup, password reset and reminder emails in user language too,
small reorganisation of lang_api - added methods lang_push(...), lang_pop(), lang_set(...);
implemented lazy loading of languages.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2475 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9