umockdev-run: Propagate signals to child

As we now keep the wrapping umockdev-run process, propagate signals like TERM
or QUIT to the child instead, so that we don't just kill the wrapper.

Also, ensure that we always clean up our testbed even if the child dies with a
signal (raising the signal does not invoke destructors).
Disable automake portability warnings

automake 1.13 enables portability warnings by default now, causing errors with
gtk-doc 1.19:

  automake: warnings are treated as errors
  docs/gtk-doc.make:77: warning: GTK_DOC_V_SETUP_$(V: non-POSIX recursive variable expansion
  docs/reference/Makefile.am:90:   'docs/gtk-doc.make' included from here
  docs/gtk-doc.make:78: warning: GTK_DOC_V_SETUP_$(AM_DEFAULT_VERBOSITY: non-POSIX recursive variable expansion

and so on. Disable the portability warnings so that autogen.sh works with these
versions.