Output path arguments should be conformed with `filepath.ToSlash`

[pauln]

In order to prevent mishaps (especially on Windows), it'd be best to run the --pbout and --svcout arguments through filepath.ToSlash (once they've been found to be non-empty) in order to ensure that all slashes are in the correct direction (/ rather than \). Accidentally providing Windows-style slashes currently results in warnings such as:

?[33mWARN?[0m[0000] Code formatting error, generated service will not build, outputting unformatted code  ?[33merror?[0m=26:20: unknown escape sequence (and 2 more errors)

This is because you end up with mixed-slashes imports (foo\bar\baz/foobar/barbaz) in the generated Go code, which Go (understandably) doesn't like.

[pauln]

This would also mean that the File Placement section of TUTORIAL.md could be simplified to say that the paths are relative to your GOROOT.