upd7810: remove gamemaster hack for INTFE1

The extended timer has been properly implemented in upd7810.
Tested with Continental Galaxy 2020.
upd7810: treat NMI like other interrupts
upd7810: correctly deal with external interrupts
upd7810: return meaningful value when PC3 is in control mode

When PC3 is in control mode, it acts as INT2 and external Timer Input.
INT2 may be set by connected devices through set_input_line(), so its
value can be used when reading or writing to PC3. There is currently
no code to support external timer input.
Merge pull request #33 from ramiropolla/upd7810

upd7810: interrupt-related fixes