Improvements to c64_0.oph startup/teardown code

16 additional bytes from the zero page are now available; the ZP
locations from $02-$8F are now free for your program's use.

Approximately 128 additional bytes in main RAM are now available,
giving you free reign from $0800-$CFFF. Zero Page backup is now
handled underneath the KERNAL's ROM, with the program epilogue
safely handling swapping out the KERNAL for the duration of the
switch. (IRQs are disabled, and NMI handling code is replicated
and modified to not hurt anything.)

Program exit is now handled by keyboard buffer and jumping
through BASIC's warm reset vector. This technique lets programs
play more nicely with PUCRUNCH and onefilers (which otherwise
often confused BASIC as the BASIC prologue would change as part
of decompression or link-loading).