Fix execution of /proc/self/exe within child processes

Fix execution of /proc/self/exe within child processes

* exec/exec.h (struct exec_tracee): New field `new_child'.
Also, make `waiting_for_syscall' a bitfield.
* exec/trace.c (PTRACE_GETEVENTMSG): New declaration.
(MAX_TRACEES): Bump to 4096.
(handle_clone_prepare): New function.
(handle_clone): If required, set `new_child' and wait for a
ptrace event describing the parent to arrive.
(after_fork): Clear new field.
(exec_waitpid): Upon a ptrace event describing a clone, create
the child's tracee if it doesn't already exist.  Otherwise, copy
over the parent's cmdline and start running it.