debian: Use useradd and groupadd in favor of adduser

debian: Use useradd and groupadd in favor of adduser

The adduser package is 'important', and while it is often installed,
we can't guarantee that it will always be there.  A required package,
or a package which is dragged in by a required package, such as
libuuid1, shouldn't depend on an 'important' package since that would
implicitly make it be required, which shouldn't be done unless
absolutely necesary.

So we replace the call to adduser with the lower-level useradd and
groupadd programs.  They are part of the passwd package, which is
required to be on all Debian systems.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>