Be more aggressive in preventing negative s12 and m12 for short lines.
Initialize reference argument to remquo.

Removes proj_api.h from the public API. The contents of the header file
has been moved to proj_internal.h verbatim and any references to
proj_api.h has been changed to proj_internal.h.

The documentation of proj_api.h has been removed. The only exception to
this is the API migration guides which still mention the old API.

Fixes #837

* Allow arbitrarily complex polygons in geod_polygon_*.  In the case
  of self-intersecting polygons the area is accumulated
  "algebraically", e.g., the areas of the 2 loops in a figure-8
  polygon will partially cancel.
* Simplify code by using C99 functions remainder and remquo.
* More test coverage.

Fixes to associated files:

* src/pipeline.cpp invoke geod_init with f = es / (1 + sqrt(1 - es))
  instead of (the less accurate) f = 1 - sqrt(1 - es)
* src/apps/geod_set.cpp remove "#undef f" (a dangling relic?).