small fixups

- remove -c abbrv. for --vips-concurrency
- better desc for insert
- revise vips.1 man page
doc fixups
better --vips-progress feedback for dzsave
png read fixes

- call png_read_end()
- catch errors during header read
- free read mem sooner
bump version to 7.40

and rename header and edvips as vipsheader and vipsedit

remove mergeup and find_mosaic
fix make dist
update man pages

oops, forgot to update man pages for header -> vipsheader rename
fix guess prefix

oop, basename / dirname swapped
save openslide metadata to .dzi files

see https://github.com/jcupitt/libvips/issues/137

todo:

- save to zoomify ImageProperties.xml as well?
- any other useful fields?
stop dzsave zip write at 4gb

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

not great, but better than producing a corrupt file
write props to a separate file

and dump all vips props
wrap props in another element
note date and version in dzprops
fix a bash-ism in configure.ac

helps freebsd
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