#2 Fix endianness bugs

[Andrew Pam]

gpasman 1.9.3 doesn't work correctly on big-endian
hardware such as ppc. The existing code that tries to
byte-swap when writing the file was unfortunately never
#defined, because ideally the files would always be
written in network byte order (big-endian). There are
standard functions htons() and ntohs() that should be
used for this. Sadly, since the existing code was
never actually used, existing gpasman data files are
always little-endian.

Therefore it is now necessary to support this by making
gpasman byte-swap the data when loading data files on a
big-endian architecture, and continue to always write
files as little-endian for compatibility.

[Andrew Pam]