Fix breakage due to iperf.h depending on the autoconf config.h file but
not including it.

To fix this required us to change config.h to iperf_config.h (to
avoid potential filename collisions with this generic name).  Then
iperf.h could include this.

Adjust the existing header file inclusions to track this, and also
canonicalize their inclusion to be at the top of *.c files.