Relocated the Win32 style file finder to libdeng1

Updated the code for modern conventions, and added some Str utility
functions.
Unix: Relocated strupr/strlwr to libdeng1
Merge branch 'master' into separate-server
Relocated dd_snprintf to libdeng1
Defined new API exchange mechanism

The plugins must export an entry point called deng_API() that the
engine will use to pass API structs to.

Todo: Engine must call deng_API to supply all available public APIs
when loading a library.
Publish the Uri module and W APIs

The Library loader now calls deng_API() to publish all the public
APIs.

Added api/de_api.h for the generic/base API struct and macro
declarations.