units, fuzz: unify macro variables used in fuzz and units targets

FUZZ_ and UNITS_ prefixes used in those targets are removed.

Following macro variables are referred in those targets:

    * VG (units, fuzz)
    * SHRINK (units, fuzz)
    * TIMEOUT (units, fuzz)
    * LANGUAGES (units, fuzz)
    * CATEGORIES (units)
    * UNITS (units)

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
docs: reflect the latest development of misc/units to documents

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