Convert admon graphics to SVG.

Use a common set of SVG files for AsciiDoc / DocBook admonition
graphics. Put them in a common directory. According to
http://caniuse.com/svg all common browsers have had SVG support for
a while now.

The graphics themselves were created with Inkscape. If you would like
to refine them further you are more than welcome.

Use variables to assemble xsltproc commands in Autotools and Nmake
while we're here.

Try to update Debian rules to reflect ga92c3fb.

Change-Id: If82647af27a60117c517125dff0aca81c033be72
Reviewed-on: https://code.wireshark.org/review/3206
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>