Make sure that "PIDFile" option cannot be set when command-line option "-P" was used. (Fixes #553)

The "PIDFile" value from command-line option "-P", which should take
precedence over any "PIDFile" value from configuration files, could be
overwritten if a configuration file contained the "PIDFile" option.

We introduced a new global variable "pidfile_from_cli" which will be set
to 1 when the command-line option "-P" was detected and the
"global_option_set" function will only set the "PIDFile" option if
"pidfile_from_cli" is 0.
Merge pull request #553 from Whissi/issue_553

collectd ignores command line argument "-P"