drcov2lcov fails to handle code in ELF segments past a gap

Xref #2213 (closed). drmodtrack now supports ELF libraries with gaps, but while drcov2lcov will now not think one library covers other libraries inside it, it will miss code present in other than the first segment. It creates a hashtable by path and stores the size, which is now just the 1st segment size. We can't simply change the table to store by start address, though, b/c we'd have to redo enumerate_line_info which walks the htable and calls drsym_enumerate_lines() on each path, passing a single bitmap. It will take some work to support multiple segments.