utmp.c to chowntty.c. checkutmp() now only uses an existing utmp
entry if the pid matches and ut_line matches with the current tty.
This fixes a possible DOS when entries can be forged in the utmp
file.
* libmisc/chowntty.c, src/login.c, lib/prototypes.h: Remove the
tty argument from chown_tty. chown_tty always changes stdin and
does not need this argument anymore.