Fix a memory leak in `libelftc`.

Ticket:	#367

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2867 95820547-d848-0410-985e-9ae8fe0fa350
Update the `libelftc` test suite:
 * Correct a test case message.
 * Test the `elftc_string_table_from_section()` API.

Ticket: #361

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2868 95820547-d848-0410-985e-9ae8fe0fa350
Update `libelftc`:
 * Implement the `elftc_string_table_from_section()` API.
 * Correct a few signed/unsigned comparisions.

Ticket:	#367

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2869 95820547-d848-0410-985e-9ae8fe0fa350