Also build gost-engine in library form

In this form, the GOST engine isn't loadable through OpenSSL's dynamic
ENGINE loader, but directly as its own function, ENGINE_load_gost().
After making that call, the engine functionality can be used as usual.
This includes a public header file called gost-engine.h, which declares
that functions.

This also rearranges the code in gost_eng.c, as the binding
functionality was spread around in the file.  Now, it's all nicely
tucked at the end.