speedo: Get version numbers from online database.

* build-aux/getswdb.sh: New.
* build-aux/speedo.mk: Get release version numbers from swdb.lst.

This should make maintaining GnuPG installations easier. Running

make -f /foo/gnupg/build-aux/speedo.mk TARGETOS=native WHAT=release

downloads all GnuPG related packages and builds them. The gnupg
directory may be a GIT checkout but in that case please run
./autogen.sh on it first. Note that currently swdb.lst is always
downloaded from gnupg.org and thus monitoring the network or the gnupg
machine reveal information on who is currently building GnuPG. If
there is an easy way to detect that TOR is enabled this can be changed
to directly download from the GnuPG hidden service.