Audit logname(1)

1) Add usage().
2) Idiomatic argv0-setter. We don't use arg.h, as we do not process
   flags or arguments.
3) Remove program-name from eprintf-call. This is done in the eprintf-
   function itself when the DEBUG-define is set.
   We'll activate it by default later.
4) Add empty line before return.

README

@@ -41,7 +41,7 @@ The following tools are implemented ('*' == finished, '#' == UTF-8 support,
 =*| link            yes                          none
 =*| ln              yes                          none
 =*| logger          yes                          none
-=*  logname         yes                          none
+=*| logname         yes                          none
 =   ls              no                           (-C), -S, -f, -m, -s, -x
 =*| md5sum          non-posix                    none
 =*| mkdir           yes                          none