dtrace: do not overload libproc flags

dtrace stored its PR_RLC and PR_KLC flags in the proc_handle's flags,
where they collided with PATTACH_FORCE and PATTACH_RDONLY, respectively.
Thus, Psetflags(PR_KLC) effectively also set the PATTACH_RDONLY flag.

Since the flags are private to dtrace (at least on FreeBSD), store them in
dtrace's own dt_proc structure instead.

On FreeBSD, either PR_RLC or PR_KLC was always set, so remove code that
would handle the case where neither was set.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D41121