Metasploit::Framework::Framework::Pathnames

MSP-9228

Msf::Config is left with only paths under the config directory, so
change its port Metasploit::Framework::Framework::Configuration to
Metasploit::Framework::Framework::Pathnames.  The instance is frozen at
the end of #intialize so that module instances accessing it from other
threads don't have to worry about shared mutable state.  Not yet
connected to Msf::Framework.