hk file extension numbers in xselect

Change requested by Suzaku XIS team to allow multiple extensions in HK files. The read hk command now allows an extension to be given eg filename+3. This entire name plus extension is passed to maketime when doing a select hk. The code change does in principle allow the same syntax to be used when reading an event file however this is disabled because in some places xselect appends a +1 extension descriptor to the event file name.