stop dzsave zip write at 4gb

see https://github.com/jcupitt/libvips/issues/138

not great, but better than producing a corrupt file
small fixes to help freebsd
improve short option name handling

we were setting short option names for deprecated options causing
confusion, see:

https://github.com/jcupitt/libvips/issues/110

thanks bgilbert
add --enable-docs configure option

so you can now stop all docs installing, freebsd likes this
bump soname

see https://github.com/jcupitt/libvips/issues/139
oops
silence some compiler warnings
we assumed GType fitted in an int

which isn't true on freebsd, and perhaps others
fix interlaced thumbnails

we need to cache the thumbnail before we write it in case the writer
needs to make several passes

see https://github.com/jcupitt/libvips/issues/140
Merge branch 'master' into dzsave-metadata

Conflicts:
	libvips/foreign/dzsave.c
more tweaks to dzsave props

add docs too