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.

This is essentially #771, but against devel branch. I hope I haven't introduced any bugs during merge.