Bug fix: use the correct return type for a string table API.

Ticket:	#367

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2863 95820547-d848-0410-985e-9ae8fe0fa350
`libelftc` test suite: add tests for the string table APIs.

Ticket:	#361

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2864 95820547-d848-0410-985e-9ae8fe0fa350
In `libelftc`:
 * Implement the `elftc_string_table_image()` function.
 * Simplify a few macros.

Ticket:	       #367

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2865 95820547-d848-0410-985e-9ae8fe0fa350
In `libelftc`: update the documentation for the `elftc_string_table_image()`
API to match the code.

Ticket:	#367

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2866 95820547-d848-0410-985e-9ae8fe0fa350