Fix bootstrap on Mono

Bootstrapping on Mono results in various errors and warnings:

1. The reference for the F# library is FSharp.Core, not Fsharp.Core

2. protodir must end with a '/', otherwise FSharp.Core.dll will be
   copied to protoFSharp.Core.dll (which will not be found later on)

3. sigdata and optdata should be copied when copying FSharp.Core.dll

4. fsc-proto must be marked executable, otherwise xbuild will refuse
   to run it

5. FSharp.Core is only needed to run fsc-proto, but placing it in the
   correct directory before any of the build-proto steps silences
   quite a few warnings about the missing "FSharp.Core reference"
Merge pull request #208 from ranma42/fix_bootstrap_31

Bootstrapping F# 3.1 on a clean Mono does not work