kern: Introduce kern.pid_max_limit sysctl.

The kern.pid_max_limit will hold the PID_MAX value the kernel was
compiled with. The existing kern.pid_max sysctl can be modified and
doesn't really represent maximum PID number in the system, as there
may still be processes created with higher PIDs before kern.pid_max
was lowered.

Reivewed by:	kib, zlei
Approved by:	oshogbo
Differential Revision:	https://reviews.freebsd.org/D43077

sys/kern/kern_mib.c

@@ -754,6 +754,10 @@ SYSCTL_PROC(_kern, OID_AUTO, pid_max, CTLTYPE_INT |
     CTLFLAG_RWTUN | CTLFLAG_NOFETCH | CTLFLAG_MPSAFE,
     0, 0, sysctl_kern_pid_max, "I", "Maximum allowed pid");
 
+SYSCTL_INT(_kern, OID_AUTO, pid_max_limit, CTLFLAG_RD,
+    SYSCTL_NULL_INT_PTR, PID_MAX,
+    "Maximum allowed pid (kern.pid_max) top limit");
+
 #include <sys/bio.h>
 #include <sys/buf.h>
 SYSCTL_INT(_debug_sizeof, OID_AUTO, bio, CTLFLAG_RD,

sys/sys/proc.h

@@ -955,7 +955,7 @@ MALLOC_DECLARE(M_SUBPROC);
  * in a pid_t, as it is used to represent "no process group".
  */
 #define	PID_MAX		99999
-#define	NO_PID		100000
+#define	NO_PID		(PID_MAX + 1)
 #define	THREAD0_TID	NO_PID
 extern pid_t pid_max;