bus: correct a number of gcc format string attribute usages
bus: driverd; add missing format string parameter

This was causing a nasty coredump
bus: fix a couple of format string mistakes
gitignore: add back user@.service
autogen: add shortcut for building with clang

For a while I have been cleaning up warnings when building with clang.
There are currently only two sources of warnings left: Wcast-align and Wgnu.
I am not convinced that fixing up those up is feasible so I run with them
disabled to spot regressions. E.g. clang is a bit more strict wrt to unused
variables with the cleanup attribute and I have fixed a number of those since.

Like the other options in autogen.sh I have a shortcut for clang as well. I use
'l' for llvm.
autogen: add shortcut for running scan-build

scan-build is a static analyzer in llvm. As ususal static analyzers
tend to mostly find theoretical bugs in software that has been in
production for a while. For in-development code it can be useful to
check if new issues is added as there is a chance to spot real problems
before release. For systemd we are now down to 297 issues - the vast
majority are false positives because the tool does not understand the
cleanup attribute.

Running clang's static analyzer scan-build is a bit messy. You have to
run both configure and make "inside" the build-scan tool. To have an
easy shortcut from autogen.sh I thus call both directly from it. This
makes it different from the other options in autogen.sh. I chose 's'
for static analysis.

scan-build is in the package clang-analyzer on fedora.