fix 16-bit PNG save

see

https://github.com/jcupitt/libvips/issues/156
rename vips_init() as VIPS_INIT()

stops a deprecation warning from gtk-doc

vips_init() is now a compat macro
more vips_check_init() out of API

now private
fix up vips_version*() docs
fix dzsave date attr on Windows

see

https://github.com/jcupitt/libvips/issues/157

thanks John
more docs work
more docs argh
note
Merge branch '7.40'

Conflicts:
	TODO