Doc comment [ci skip]
WIP
Simplify BufferWithOpts implementation
Fix join and cap consts
Fix Makefile
Fix double-free and pointer ownership issues

This changeset should make the library threadsafe with respect to C threads.
It follows the GEOS C API's documentation regarding ownership and freeing of
geometry and coordinate sequence pointers.

The stress test tool (continually running the test suite under varying
settings of GOMAXPROCS) ran for many minutes without any SIGABRTs
(double-frees in the underlying C library) or deadlocks.
Merge branch 'issue-5/threadsafe'