agetty: cleanup argv parsing, utmp update, use writeall(), etc.

agetty: cleanup argv parsing, utmp update, use writeall(), etc.

Next cleanup, that is split off special character parsing in do_prompt()
and parse_args(), handle name of line of virtual console and use it in
utmp update as well as the session id.  Also move the initial TERM
variable to open_tty() as we use this later in open_tty().  Use writeall()
but avoid repeat to often for EAGAIN (compare with info page of the
(g)libc, description of the macro EAGAIN).

Signed-off-by: Werner Fink <werner@suse.de>