procstat_getptlwpinfo(..): clarify the fact that KVM/SYSCTL support

isn't supported

This will make the error message reported in bug 220023 a bit more
intuitive for end-users that don't have access to the source code to
decode the procstat->type argument.

MFC after:	1 month
MFC with:	r316286
PR:		220023

lib/libprocstat/libprocstat.c

@@ -2510,6 +2510,12 @@ struct ptrace_lwpinfo *
 procstat_getptlwpinfo(struct procstat *procstat, unsigned int *cntp)
 {
 	switch (procstat->type) {
+	case PROCSTAT_KVM:
+		warnx("kvm method is not supported");
+		return (NULL);
+	case PROCSTAT_SYSCTL:
+		warnx("sysctl method is not supported");
+		return (NULL);
 	case PROCSTAT_CORE:
 	 	return (procstat_getptlwpinfo_core(procstat->core, cntp));
 	default: