proxy/handler: reorganize program startup a little bit

[jkarneges]

This tweaks a few organizational bits around proxy and handler startup:

• Functions {proxy,handler}_main are renamed to {proxy,handler}_init, to clarify they don't take argc + argv.
• Modules {proxy,handler}cliargs are renamed to cliargs since they are already scoped by program module.
• Files app.cpp and engine.{h,cpp} are renamed to proxyapp.cpp and proxyengine.{h,cpp}, respectively, for consistency with the naming of handler files.
• The content of main.cpp (for proxy) and handlermain.cpp are moved into {proxy|handler}app.cpp, respectively, and the files are removed. Relatedly, app.h (for proxy) and handlerapp.h are removed.

For background, the reason we have some program-prefixed filenames despite the files living in different directories, such as src/proxy/proxyapp.cpp and src/handler/handlerapp.cpp instead of more simply src/proxy/app.cpp and src/handler/app.cpp, is because qmake requires all files in the project to have unique names regardless of directory. If we ever find a way to overcome that limitation, we could remove these prefixes.