units: define WITH_VALGRIND at the top level

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
units: introduce --colorized-output option

In the case that escape sequences in the output is not desired,
the option can be used.

New global variable COLORIZED_OUTPUT is introduced. _CONNECTED_TO_FILE
is replaced with it.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>