split INTRNG main file into two files

[ehem]

Hopefully SSIA. INTRNG presents multiple interfaces and tries to hide some. There seems to be a border between the PIC side and the event side. Split these two apart.

The PIC side bears fair resemblance to what is in x86/nexus.c. I suspect it might be possible to have this take over some of what is done for x86. Theory is to shrink the architecture specific bits.