Make sure config.h is included before any system headers

Make sure config.h is included before any system headers

There was an issue recently in screen-share.c where config.h was not
being included, resulting in the wrong definition for off_t being used on
32 bit systems. I checked and I don't think this problem is happening
elsewhere, but to help avoid this sort of problem in the future, I went
through and made sure that config.h is included first whenever system
headers are included.

The config.h header should be included before any system headers, failing
to do this can result in the wrong type sizes being defined on certain
systems, e.g. off_t from sys/types.h

Signed-off-by: Andrew Wedgbury <andrew.wedgbury@realvnc.com>