Fix maintainer-upload-docs target to work out-of-tree

Fix maintainer-upload-docs target to work out-of-tree

The STATIC_DOCS, DTDS and all dist_ files except XMLTO_OUTPUT are in the
source tree. The XMLTO_OUTPUT and the man2html output are in the build
tree, and the BONUS_FILES already have $(srcdir) in their names.

Also change the rules that generate the dbus-docs directory so that if
they fail, they leave behind a temporary directory, rather than leaving
behind a dbus-docs directory that causes make to not run those rules
if re-run; and change the rules to scp files to the server, to put a
trailing "/" on paths, ensuring that the tarball won't be uploaded as
"www" if the www directory doesn't already exist.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=41047
Reviewed-by: Will Thompson <will.thompson@collabora.co.uk>