allow sendmail_path php directive to work

Jesse Norellrequested to merge
jnorell/ispconfig3:custom_sendmail_path into develop

Checks php directives for sendmail_path so it doesn't get added twice. (Tested with apache2 + php-fpm only.)

Fixes #5641 (closed) (does not fix the overall issue in #2118).

Edited by Jesse Norell

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