Allow sendmail options in SENDMAIL_PATH

[msebald]

• Gitea version (or commit ref): 1.2.0+rc3-7-g74399f3
• Git version: 2.11.0-3~bpo8+1
• Operating system: Debian Jessie Stable amd64
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant

Description

Please allow to set sendmail options like "-t -i -f webmaster@domain.tld" in option SENDMAIL_PATH. The reason is simple: Without options sendmail will use username@mailserver (e.g. git@mail.domain.tld, username = username, Gitea runs on the server) as a Return-Path in the outgoing mail. This can produce various problems.