OS X and iOS: Search for openssl libs in .app/Contents/Frameworks

This allows apps to ship their own openssl libraries inside the
application bundle.

The change consists of two parts: First, adding

  <bundle>/Contents/Frameworks

to the alternative search paths. Second, disabling the preemtive
check for libssl.dylib, libcrypto.dylib in the system paths:
The system's /usr/lib/libssl.dylib, /usr/lib/libcrypto.dylib
will still be picked up as a fallback in the alternative search
paths, but only after .app/Contents/Frameworks have been inspected.

[ChangeLog][QtNetwork][QSsl] On OS X and iOS, openssl dylib's are
picked up from the 'Frameworks' directory of the app bundle.

Change-Id: I982930f4a6cf5e0114c04ecbc87f27e54ba8bb88
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>