linux(4) prctl(2): Implement PR_[GS]ET_DUMPABLE

Proxy the flag to the roughly analogous FreeBSD procctl 'TRACE'.

TRACE-disabled processes are not coredumped, and Linux !DUMPABLE processes
can not be ptraced. There are some additional semantics around ownership of
files in the /proc/[pid] pseudo-filesystem, which we do not attempt to
emulate correctly at this time.

Reviewed by: markj (earlier version)
Differential Revision: https://reviews.freebsd.org/D27015