Add algorithm enum string conversion

Similar nlopt_get_* functions in options.c always check for
(opt && (opt->n == 0 || argument)), requiring a valid nlopt * opt
and only allowing a null argument when the optimizer is "empty".

nlopt_get_xtol_abs seems to have been left out (13e0f277).

* CMake: Add NLOPT_VERSION in config

* Fix few more warnings

c89 compatibility

* Add -Wall and -Wextra, fix lots of warnings

 - remove set but not used variables
 - (void) cast unused function arguments
 - create internal function duplicates for some actually
   deprecated functions, for the f77 files disable
   the deprecation warning completely.
 - add warning flags to travis

Note: some of the maybe-unitialized warnings do not
  show in gcc 7.3.0, but travis seems to use a different one.

This closes #226.

* eliminate internal usage of deprecated apis

Fixes #249

Fixes #239