fix host vs. makefile directory separator mess

fix host vs. makefile directory separator mess

the system path separator and shell are bound to the host system
(system() will use cmd even on mingw with sh.exe in path).
the makefiles otoh may depend on what the qmakespec defines.

consequently, add $$system_path() and $$system_quote() (for use with
system() & $$system()). $$native_path() is renamed to $$shell_path() and
should be used with $$shell_quote() to produce command lines in
makefiles.
$$QMAKE_DIR_SEP needs to be applied to Option::dir_sep right after
parsing the spec, so it is available to $$shell_{path,quote}().

Change-Id: If3db4849e7f96068cf03a32348a24f3a72d6292c
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>