Rename exec_program() -> win_exec_cmd()

It reflects content of the function better.
Rename get_command_name() -> extract_cmd_name()

After time has passed, I did expect the function to have this name and
spent some time figuring out its real name.
Add get_full_cmd_path() function

Unused for now.
Separate platform-specific general utilities
Remove res-file in clean target on Windows

It's a compiled rc file.
Don't wait finish of GUI applications on Windows

Check subsystem stated in PE-header of the executable to suppress
pausing for GUI-applications.

Thanks to Robert Sarkozi.
Document not waiting for Windows GUI apps
Merge branch 'win-check-gui-apps'