drmodtrack fails to handle non-contiguous ELF libraries

The drmodtrack extension, evolved from drcov's library tracker, completely ignores gaps and the possibility of non-contiguous ELF mappings. Now that drmodtrack is used for drcachesim this is being used in more places and the gap issue is now a blocker. For drcachesim, one app has its ld.so with a big gap containing other libs inside it, and drmodtrack ends up thinking that every lib in between is actually in ld.so. This leads to crashes in drraw2trace post-processing.