The intent is to make 'clone' behave a lot more like 'primitive-fork', which
calls clone(2) with SIGCHLD, CLONE_CHILD_CLEARTID, and CLONE_CHILD_SETTID
flags. Notably, running 'clone' at the REPL without these flags would break
the REPL beyond repair.
* guix/build/syscalls.scm (CLONE_CHILD_CLEARTID, CLONE_CHILD_SETTID): New
variables.
* gnu/build/linux-container.scm (namespaces->bit-mask): Add
CLONE_CHILD_CLEARTID and CLONE_CHILD_SETTID to bit mask.