Fix problem with images referenced within svg files. (bug#19373)

Fixes: debbugs:19373

* src/image.c: Additional parameter 'filename' for
svg_load_image. Include "buffer.h".  Define library function
rsvg_handle_set_base_uri for WINDOWSNT.
(init_svg_functions): Initialize rsvg_handle_set_base_uri.
(fn_rsvg_handle_set_base_uri): Define fn_rsvg_handle_set_base_uri.
(svg_load): Pass a filename to svg_load_image: either name of
actual file or of current buffer's file.
(svg_load_image): New parameter 'filename', used for setting
base_uri, necessary for loading referenced images.
Introduce tramp-histfile-override.

* net/tramp-sh.el (tramp-histfile-override): New variable.
(tramp-open-shell, tramp-maybe-open-connection): Use it.
Merge branch 'master' of git.sv.gnu.org:/srv/git/emacs
Partially disabled previous change.

* src/image.c (svg_load): Temporarily disabled filename thing for
not-a-file case as it can cause crashs.