Cygwin: open_shared: try harder allocating a shared region

Cygwin: open_shared: try harder allocating a shared region

For fixed regions (cygwin/user/myself/shared console), try fixed
address first.  Fallback to non-fixed region.  Don't even try fixed
address if the Cygwin DLL gets dynamically loaded.

For non-fixed regions, try to allocate in a loop within the area
from SHARED_REGIONS_ADDRESS_LOW to SHARED_REGIONS_ADDRESS_HIGH.

Fixes: 60675f1a7eb2 ("Cygwin: decouple shared mem regions from Cygwin DLL")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>