MFC r273837:

Fix a clang 3.5 warning about abs(3) being given an argument of type
quad_t in setusercontext().  While here, sanitize the clamping of the
priority value, and use the correct type for the return value of
login_getcapnum().

Reviewed by:	kib
Merge remote-tracking branch 'freebsd/stable/10' into hardened/10/aslr