Bug 9683 – RDMD: Emit error on missing or unexpected main function

After https://github.com/D-Programming-Language/dmd/pull/1732 is pulled RDMD should be able to the following: 1) Emit an error on missing main if 'entry' is not found. This will avoid displaying linker error messages. 2) When RDMD supports building libraries (e.g. using the -lib or maybe --lib/--static switch) it should warn or error if a 'main' function is found. This will avoid linker errors for multiply defined 'main' symbols when the user links the built static library with his code at a later point. 3) When RDMD supports building DLLs it should verify there's an entry point and that the entry point is dllmain.

Actually this can only work if there are no static library or object files passed on the command line to RDMD, since they could potentially contain 'main.'.