Introduce the new function

p_trespass(struct proc *p1, struct proc *p2)

which returns zero or an errno depending on the legality of p1 trespassing
on p2.

Replace kern_sig.c:CANSIGNAL() with call to p_trespass() and one
extra signal related check.

Replace procfs.h:CHECKIO() macros with calls to p_trespass().

Only show command lines to process which can trespass on the target
process.