Rename ENV variable, keep backward compatibility

Rename NAGIOS_PLUGIN_STATE_DIRECTORY to MP_STATE_DIRECTORY
Clean up parse_ini error messages, add missing newlines
Ignore MP_STATE_DIRECTORY in suid plugins

If a plugin still has suid privileges at the time np_enable_state() is
called, the MP_STATE_DIRECTORY environment will be ignored.

There is no need for a NEWS entry as no suid plugins use np_enable_state
yet.