Check result of fgets() and sscanf() in wxGetFreeMemory().

This fixes g++ -Wunused-result warnings and also actually makes the code more
robust.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76530 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Use wxMemorySize type in Unix implementation of wxGetFreeMemory().

This should avoid overflows on 32 bit systems with more than 2GB of RAM.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76531 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775