Added a new "preserve" var for build templates.

When a package has this defined, its files won't be removed after
a removal or upgrade process. This should be used in a few cases,
mostly for the kernel pkg to always preserve a working kernel+modules.

--HG--
extra : convert_revision : 03b19d1df2b029b6671865072b4d9529b0b86aa4

xbps-src/shutils/metadata.sh

@@ -289,6 +289,12 @@ _EOF
 		echo "<true/>" >> $TMPFPROPS
 	fi
 
+	# pkg needs to preserve its files after removal/upgrade?
+	if [ -n "$preserve" ]; then
+		echo "<key>preserve</key>" >> $TMPFPROPS
+		echo "<true/>" >> $TMPFPROPS
+	fi
+
 	# Dependencies
 	if [ -n "$run_depends" ]; then
 		echo "<key>run_depends</key>" >> $TMPFPROPS