write: remove unnecessary utmp variables

write: remove unnecessary utmp variables

glibc documentation tells getutent() calls are not thread safe, and
recommends to copy the context of the structures when information is wished
to be stored.  This leads to excessive copying, that in this case is not
relevant.  write(1) is single threaded program and there is no reason to
assume this would change in future.

Reference: http://www.gnu.org/software/libc/manual/html_node/Manipulating-the-Database.html
Signed-off-by: Sami Kerola <kerolasa@iki.fi>