split surf_parse into a separate file to make it reusable from elsewhere

split surf_parse into a separate file to make it reusable from elsewhere

Also rename surf_parse_* functions into xbt_parse_*

Also, make the file name and line number explicit, as parameters to
functions that need it (less hidden global state is better). But I was
too lazy to clean smpi::parse_factor() tonight: the pandora box
remains closed for tonight.