Revamp -f support for Linux.
* util.c [LINUX] (setbpt, clearbpt): New implementations that tweak
the system call to be clone with CLONE_PTRACE set. Various new static
helper functions.
* process.c (internal_clone): Define also #ifdef SYS_clone2.
Initialize TCPCHILD->parent field.
[CLONE_PTRACE]: Don't do PTRACE_ATTACH here, because it's preattached.
Check in case the new child is in the tcb already.
(internal_fork) [LINUX]: Just call internal_clone.
* strace.c (trace) [LINUX]: Under -f/-F, grok an unknown pid
reporting to wait, put it in the TCB with TCB_ATTACHED|TCB_SUSPENDED.