agetty: use getttynam() if available

agetty: use getttynam() if available

/etc/ttys seems to be a rather archaic concept that is not meant to
exist on Linux. Nevertheless it does. glibc has getttynam() which
correctly parses /etc/ttys. So let's give it a try before falling back
to the built in defaults. One can set the terminal type for a
specific tty using e.g.:

echo 'ttyS0 "" xterm' > /etc/ttys

[kzak@redhat.com: - improve configure.ac part
                  - log error on failed strdup()]

Signed-off-by: Karel Zak <kzak@redhat.com>