Discontinued distributing libpng-1.7.0-1.6.0-diff.txt and similar.

code to the hardware subdirectory and added comments to pngrutil.c so that
implementors of other optimizations know what to do.

png_handle_unknown() when SAVE support is absent.  Moved the shell test
scripts for contrib/libtests from the libpng top directory to contrib/libtests.
png_handle_unknown() must always read or skip the chunk, if
SAVE_UNKNOWN_CHUNKS is turned off *and* the application does not set
a user callback an unknown chunk will not be read, leading to a read
error, which was revealed by the "tunknown" test.

issues with 1.12 and insisting on anything less would allow 1.12 to be used.

generating the linker mapfile symbol prefix.

Renamed internal configure variable from prefix_TRUE to DO_PNG_PREFIX_TRUE

to be prefixed (suggested by Reuben Hawkins).  This is work in progress that
breaks some non-configure builds.