security(7): fix mandoc -T lint complaints

- Add `.Nm` section for securelevel(7) to match corresponding MLINKS entry.
- Fix the spelling for mac(4) (the actual subsystem manpage is spelled out in
  lowercase.

MFC after:	1 week

share/man/man7/security.7

@@ -30,7 +30,8 @@
 .Dt SECURITY 7
 .Os
 .Sh NAME
-.Nm security
+.Nm security ,
+.Nm securelevel
 .Nd introduction to security under FreeBSD
 .Sh DESCRIPTION
 See
@@ -543,7 +544,7 @@ may not be loaded or unloaded.
 The kernel debugger may not be entered using the
 .Va debug.kdb.enter
 sysctl unless a
-.Xr MAC 9
+.Xr mac 9
 policy grants access, for example using
 .Xr mac_ddb 4 .
 A panic or trap cannot be forced using the