Userspace fexec

Removes kernel implementations of SYS_FEXEC and SYS_CLONE, replacing them with fork and execve implementations in relibc (as well as in escalated for setuid/setgid). Also adds necessary interfaces for reusing references of, or copying, address spaces, file tables, and sigactions, as well as for setting e.g. name, cwd, uid, gid, etc.

Also removes initfs scheme, moving it to bootstrap (and a tiny part of bootloader).

And finally, and perhaps most exciting, phases out the old paging code in favor of RMM.