Remove unneccesary header files.

Remove unneccesary header files.
(__libc_utmp_unknown_functions): Added getutid_r_unknown,
getutline_r_unknown.
(setutent_unknown): Only set file backend if setutent for the file
backend was successful.
(getutent_r_unknown, pututline_unknown): Call setutent_unknown instead
of __setutent.  Report failure if no backend was selected.
(getutid_r_unknown): New function.
(getutline_r_unknown): New function.
(__endutent): Reset backend.  This makes sure all backends are checked
on the next setutent call.