Encode literal global characters in files.  Otherwise the results of
updating a directory using the resulting spec will be surprising.

See http://svnweb.freebsd.org/base/head/usr.sbin/mtree/test/test00.sh
for test cases.
Index: spec.c
===================================================================
--- spec.c	(revision 240788)
+++ spec.c	(revision 240789)
@@ -415,7 +415,8 @@
 char *
 vispath(const char *path)
 {
-	const char extra[] = { ' ', '\t', '\n', '\\', '#', '\0' };
+	const char extra[] = { ' ', '\t', '\n', '\\', '#', '*', '?', '[',
+	    '#', '\0' };
 	static char pathbuf[4*MAXPATHLEN + 1];
 
 	strsvis(pathbuf, path, VIS_CSTYLE, extra);