ubuntu: Treat Ubuntu as a distinct distro in configure.ac etc

Previously Ubuntu was treated as being equivalent to Debian, but the two
distributions require different behaviour in certain places.  This commit does
not change the behaviour of systemd on either distro but it creates a
framework for changes to be introduced by later commits.

The following previously meant "Target is Debian or Ubuntu".
    * configure option "--with-distro=debian"
    * C preprocessor symbol "TARGET_DEBIAN"
    * Automake conditional "TARGET_DEBIAN"
After this commit, all of the above are redefined to mean "Target is Debian"

The following are introduced to mean "Target is Ubuntu".
    * configure option "--with-distro=ubuntu"
    * C preprocessor symbol "TARGET_UBUNTU"
    * Automake conditional "TARGET_UBUNTU"

Most code written for Debian will also be applicable to Ubuntu. An extra
Automake conditional "TARGET_DEBIAN_OR_UBUNTU" is introduced to avoid
duplication of code that would otherwise occur.

This commit updates configure.ac, Makefile.am and distro-specific source files
in line with the above definitions.