Explicitly set the umask for daemon

As discovered by danielbeck in #jenkins, daemon(1) does not inherit the parent
process's umask. The --umask flag is the means to set it explicitly.
only set umask if set in /etc/default/jenkins
remove useless call to umask
Merge pull request #1397 from OEP/fix-ignored-umask

Explicitly set the umask for daemon in Debian init script