osproc: introduce poEvalCommand, poUsePath, fix remaining quoting issues.

- `poUseShell` is now an alias for `poUsePath`.
- new `poEvalCommand` should be used when shell evaluation is really needed.
It passes `command` directly to shell/winapi. Requires `args` parameter
to be empty.

This change is incompatible with previous code (before fixing quoting) in only one (sane) case - calling shell builtin such as `help` using `poUseShell`.

**I have only partially tested it on Windows, because I don't know how to bootstrap Nimrod there.**