xv68k:  Fail noisily on code input errors

If we fail to open the file via the supplied path, abort instead of just
exiting.  Also abort if read() fails.
xv68k:  Define some things later in execute_68k()

Move things out of the way so subsequent refactoring patches are nicer.
xv68k:  Factor out load_Mac_traps()
xv68k:  Factor out load_argv()
xv68k:  Factor out load_code()
xv68k:  Factor out report_condition()
xv68k:  Factor out emulation_loop()
Merge branch 'xv68k-refactor'
xv68k:  Recognize "-" input filename as stdin
xv68k:  Call load_argv() with adjusted argc, argv
xv68k:  Detect - and -- and reject all options
v68k-syscalls:  Allow custom getpid() result
xv68k:  Add initial -A support
xv68k:  By default, use a fake PID of 12345
v68k:  Add getpid demo
Merge branch 'fake-pid'