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.

include/xbps.h.in

@@ -232,6 +232,14 @@
  */
 #define XBPS_FLAG_INSTALL_REPRO		0x00008000
 
+/**
+ * @def XBPS_FLAG_KEEP_CONFIG
+ * Don't overwrite configuration files that have not changed since
+ * installation.
+ * Must be set through the xbps_handle::flags member.
+ */
+#define XBPS_FLAG_KEEP_CONFIG	0x00010000
+
 /**
  * @def XBPS_FETCH_CACHECONN
  * Default (global) limit of cached connections used in libfetch.