Updated to make 'port92' (from mem.ch) a device in devices/system/port92.c
Several small changes and cleanups.
Big edit to replace the 'void *' arguments with a new priv_t typedef to allow better enforcing of types in future cleanup rounds.
Devices now have a 'path' member, which usually points to a ROM file, or loader script.
No more 'available' functions unless one is needed for special cases.
Machines now can be 'configured' for mainboard settings etc.
Machines are now devices as well (root devices).
Regular devices now get a pointer to a 'parent' device.
The machines table has been reduced to just a lookup table, machine info is now inside the machine file.
Machine-specific CPU lists are now inside the machine file.
Chipset implementations are now devices, and were moved to devices/chipsets/
Much work on individual machines done.
All COMPAQ machines moved to dev-branch.
Zenith SupersPORT moved to dev-branch.
The i686-based machines (Tyan) released from dev-branch and cleaned up.
Several new MFM and RLL disk controllers for XT, in dev-branch for now.
