src/useradd.c: E_BAD_NAME: Use a different error code for bad login names

src/useradd.c: E_BAD_NAME: Use a different error code for bad login names

Wrappers like adduser(8) want to do their own stuff if the login name is
bad.  For that, they need to be able to differentiate such an error.

Closes: <https://github.com/shadow-maint/shadow/issues/1103>
Suggested-by: Chris Hofstaedtler <zeha@debian.org>
Cc: Marc 'Zugschlus' Haber <mh+githubvisible@zugschlus.de>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>