Adds handling of gaps inside ELF libraries to drmodtrack. Additional entries are added both to the online vector and the written file. The containing_index field added previously is used to point to the main entry from sub-entries.
It's not easy to write a test without making a linker script, but on many recent Linux distros there are gaps in some or even all system libraries, including some of my machines, so we'll get at least some future regression coverage.
Fixes #2213 (closed)