Fix various memory leaks

Fix various memory leaks

These leaks are reported when building and running the tests with
LeakSanitizer (ignoring regression tests and tests marked as integration
tests).

- Free memory on 'gmx help -export completion' paths, which also fixes
  all 'gmx help <module>' paths.
- Free all memory in genconf on exit to make the gmxpreprocess tests not
  leak memory.
- Add settle_free() to free memory properly in mdlib tests.
- Add missing sfree in done_state() (fixes leaks in read_tps_conf() for
  tpr files).
- Add missing virtual destructor in IOptionValueStore (various leaks
  from correct destructors not getting called).
- Free the final symbol in selection parser in case the parsing ends
  after the maximum number of selections has already been provided.
- Add missing frees in selection test helper for atom type strings.

Change-Id: I0d796033ce3b30cda0a18743a28f4d3133fd477f