Don't set P2_PTRACE_FSTP in a process that invokes ptrace(PT_TRACE_ME).

Such processes are stopped synchronously by a direct call to
ptracestop(SIGTRAP) upon exec. P2_PTRACE_FSTP causes the exec()ing thread
to suspend itself while waiting for a SIGSTOP that never arrives.

Reviewed by: kib
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D7576