configuration: add keepconf option

Add configuration option keepconf that stops xbps from overwriting
unchanged configuration files. If keepconf=true, xbps will store the new
configuration as <name>.new-<version> instead of overwriting unchanged
configuration files.

data/xbps.conf

@@ -71,6 +71,20 @@
 #	preserve=/etc/file
 #	preserve=/etc/dir/*.conf
 
+## PRESERVING CONFIGURATION
+#
+# The `keepconf` (disabled by default) keyword can be used to prevent
+# xbps from overwriting configuration files.
+#
+# If set to false, xbps will overwrite configuration files if they
+# have not been changed since installation and a newer version is
+# available.
+#
+# If set to true, xbps will save the new configuration file as
+# <name>.new-<version> if the original configuration file has not been
+# changed since installation.
+# keepconf=true
+
 ## VIRTUAL PACKAGES
 #
 # Virtual package overrides. You can set your own list of preferred virtual

data/xbps.d.5

@@ -1,4 +1,4 @@
-.Dd January 18, 2020
+.Dd February 05, 2020
 .Dt XBPS-D 5
 .Sh NAME
 .Nm xbps.d
@@ -84,6 +84,15 @@ Absolute path to a file and file globbing are supported, example:
 .It Sy preserve=/usr/bin/foo
 .It Sy preserve=/etc/foo/*.conf
 .El
+.It Sy keepconf=true|false
+If set to false (default), xbps will overwrite configuration files if
+they have not been changed since installation and a newer version is
+available.
+.Pp
+If set to true, xbps will save the new configuration file as
+<name>.new-<version> if the original configuration file has not been
+changed since installation.
+.Pp
 .It Sy repository=url
 Declares a package repository. The
 .Ar url