sysctl: permission check based on capability not euid

This patch modifies the permission checks for sysctl's from being based on uid=0 (root) to use the capability system. This matches the behavior of other OS's using sysctl's and capabilities. Linux has tried to get away from using uid=0 for security overrides and use capabilities instead.

I was working on Quagga enhancement that involved enabling a sysctl, and it didn't work because is a safe daemon and drops privileges and resets its real/effective uid after initialization; it then re-enables only the capabilities when it needs to do some privileged operation. This wouldn't work because sysctl's were still using the root based permission check. The existing code in quagga to enable ip forwarding doesn't work for the same reason.

---
Maybe once the BKL is gone, the last vestiges of current->euid == 0 can be wiped out as well.

@@ -261,6 +261,7 @@ typedef struct kernel_cap_struct {
    arbitrary SCSI commands */
 /* Allow setting encryption key on loopback filesystem */
 /* Allow setting zone reclaim policy */
+/* Allow setting any sysctl value */
 
 #define CAP_SYS_ADMIN        21
 

@@ -1561,7 +1561,7 @@ out:
 
 static int test_perm(int mode, int op)
 {
-	if (!current->euid)
+	if (capable(CAP_SYS_ADMIN))
 		mode >>= 6;
 	else if (in_egroup_p(0))
 		mode >>= 3;

I don't think CAP_SYS_ADMIN makes sense for net.*, for example.