Allow rtprio_thread to operate on threads of any process
This in particular unbreaks rtkit.
The limitation was a leftover of previous state, to quote a
comment:
/*
- Though lwpid is unique, only current process is supported
- since there is no efficient way to look up a LWP yet. */
Long since then a global tid hash was introduced to remedy
the problem.
Permission checks still apply.
Submitted by: greg_unrelenting.technology (Greg V)
Differential Revision: https://reviews.freebsd.org/D27158