Update the proposed string table API:
 * Partially revert changeset [2847]: have the `_insert()` and
   `_lookup()` APIs to return offsets as before.
 * Document operations that may invalidate prior offsets returned.
 * Add a function `elftc_string_table_to_string()` to convert offsets
   to pointers to interned strings when needed.
 * Add an 'IMPLEMENTATION NOTES' section describing the expected
   pattern of use, and the expected performance characteristics of the
   API.

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2857 95820547-d848-0410-985e-9ae8fe0fa350
Update function prototypes for the string table APIs, post changeset
[2857].

Ticket:	#367

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2858 95820547-d848-0410-985e-9ae8fe0fa350