Implement FreeBSD's "nochange" keyworld which causes the existance of a
filesystem object to be checked but none of its attiributes.

Index: mtree.8
===================================================================
--- mtree.8	(revision 240795)
+++ mtree.8	(revision 240796)
@@ -463,6 +463,8 @@
 value.
 .It Sy nlink
 The number of hard links the file is expected to have.
+.It Sy nochange
+Make sure this file or directory exists but otherwise ignore all attributes.
 .It Sy optional
 The file is optional; don't complain about the file if it's
 not in the file hierarchy.
Index: mtree.h
===================================================================
--- mtree.h	(revision 240795)
+++ mtree.h	(revision 240796)
@@ -86,6 +86,8 @@
 #define	F_UID		0x00080000		/* uid */
 #define	F_UNAME		0x00100000		/* user name */
 #define	F_VISIT		0x00200000		/* file visited */
+#define	F_NOCHANGE	0x00400000		/* check existance, but not */
+						/* other properties */
 #define	F_SHA256	0x00800000		/* SHA256 digest */
 #define	F_SHA384	0x01000000		/* SHA384 digest */
 #define	F_SHA512	0x02000000		/* SHA512 digest */
Index: verify.c
===================================================================
--- verify.c	(revision 240795)
+++ verify.c	(revision 240796)
@@ -124,7 +124,8 @@
 			    !fnmatch(ep->name, p->fts_name, FNM_PATHNAME)) ||
 			    !strcmp(ep->name, p->fts_name)) {
 				ep->flags |= F_VISIT;
-				if (compare(ep, p))
+				if ((ep->flags & F_NOCHANGE) == 0 &&
+				    compare(ep, p))
 					rval = MISMATCHEXIT;
 				if (!(ep->flags & F_IGN) &&
 				    ep->type == F_DIR &&
Index: misc.c
===================================================================
--- misc.c	(revision 240795)
+++ misc.c	(revision 240796)
@@ -71,6 +71,7 @@
 	{"md5digest",	F_MD5,		NEEDVALUE},
 	{"mode",	F_MODE,		NEEDVALUE},
 	{"nlink",	F_NLINK,	NEEDVALUE},
+	{"nochange",	F_NOCHANGE,	0},
 	{"optional",	F_OPT,		0},
 	{"ripemd160digest", F_RMD160,	NEEDVALUE},
 	{"rmd160",	F_RMD160,	NEEDVALUE},