Make easier for others to use; Drop __NR_jmpfwd

Also a lot of minor documentation updates and making the userspace $BATCH_EMUL interpreter look more like the in-kernel-module interpreter.

Make build more robust2

Oops - released pre-push. Sorry.

Make build more robust

Mostly with regard to care & feeding of gcc case fall through warnings, but via disabling -Werror.

Add a few kernel version portability fixes

Adapt to rename of __NR_syscall_max to NR_syscalls in Linux 5.14

Allow make V=whatever to override kernel sources to build against.

Also add BATCH_VERBOSE evar which if set to anything writes to stderr which mode is in effect.

Initial public release

This is just to version stamp things.