Audit nohup(1)

1) no need to include sys/stat.h
2) remove the enum which just added a layer too thick on this simple
   program
3) argc-style, other style
4) weprintf instead of enprintf, then save the error-message of
   execvp before and return the proper status.
5) write consistent "not reached" comment.

README

@@ -50,7 +50,7 @@ The following tools are implemented ('*' == finished, '#' == UTF-8 support,
 =*  mv              yes                          none (-i)
 =*| nice            yes                          none
 =   nl              no                           -d, -f, -h, -l, -p
-=*  nohup           yes                          none
+=*| nohup           yes                          none
 #*  paste           yes                          none
 =*| printenv        non-posix                    none
 #*  printf          yes                          none