z80pack, tricks: update bankfork

We now use all but 4K for base memory so we may need to copy more space.
Without this fix programs with large or no chmem crashes in fork()
usermem: fix error handling
usermem-z80: return 0
z80pack: put the kernel flag in common

As this is checked with the kernel not mapped it's important it lives somewhere
more suitable
process: kernel flag has moved to common
binman: memcpy the initialized data over

This accidentally got removed when I stripped it down from the full
featured kernel version.