lock for metadata changes

Another attempt at fixing crashes on metadata chenage in highly threaded
applications.

Global lock around set, remove and copy metadata. This is crude, but
simple, the performance impact should be small, and ought to resolve the
problem.

We'll do something better for the next version.

see lovell/sharp#1986

fix some autorot issues

and a problem with vipsthumbnail of tif

see libvips#1515

fix up i18n

POTFILES etc. needed updating for the recent class renames.

better error handling in vips__seek

Now that we are using the poxis API on win32, we don't need to check
GetLastError()

revise webpload for improved loader pattern

fix compiler warning

fix SANITY checks in streami.c

they had bitrotted a bit

small changes to the gif-loop fix

see libvips#1362

fix another compiler warning

always copy before exif_update

During write, we often call vips__exif_update(). This updates the exif
block from the other image metadata prior to save.

Always copy the image before calling this.

See lovell/sharp#1986