eina_list: Optimize eina_list_nth() call in case of index 0.

Check null and return the list data directly if the index 'n' is 0.

Backport from commit 13d1e5dcf0cacf537bbdaf56e2d9418b0d075211
on https://git.enlightenment.org/core/efl.git
make Eina_Error thread safe.

Backport from commit c3e0f5c4ef7922c2fe29db66e4744a987f31bff3
on https://git.enlightenment.org/core/efl.git
Fix bug in eina_convert_itoa

The current implementation of eina_convert_itoa cannot convert INT_MIN.

When the input number is negative, the function negates it and this is
an undefined behavior for INT_MIN since -INT_MIN cannot be represented
in a signed int.

Backport from commit 53e0a2d6bd726385ae9c702eb54299287af781f6
on https://git.enlightenment.org/core/efl.git
General cleanup of documentation

I've combed through the Eina source files and made enhancements to the
documentation, including:
- Document the undocumented
- Fixed some errors in Doxygen markup
- Moved some function documentation from implementation (.c or .x) to definition
  (.h)
- Edited some of the entries to improve clarity

Backport from commit cf5192025f3aa21115580aa6577eb0f12c2f6ec2
on https://git.enlightenment.org/core/efl.git