libutil/login: was totally broken. fixed

libutil/login: was totally broken. fixed

Also made login() and logout() usable with utmpname(non_std_utmp_file)
   text           data     bss     dec     hex filename
    111              0       0     111      6f libutil/login.o
    217              0       0     217      d9 libutil/login.o
   text           data     bss     dec     hex filename
    164              0       0     164      a4 libutil/logout.o
    134              0       0     134      86 libutil/logout.o

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>