bad line counts from eqn 1.23.0

[Douglas McIlroy]

eqn issues a .lf for every .EN. In 1.23.0 the line number is assigned as if
there were only one line of eqn text between .EQ and .EN, regardless of how
many lines actually are present. Thus the two fragments below yield
identical sequences of .lf requests
       .EQ                .EQ
      1                 1
      .EN   (.lf 3)     .EN   (.lf 3)
      .EQ               .EQ
      1                 1
      .EN   (.lf 6)     2
      .EQ               .EN   (.lf 6)
      1                 .EQ
      .EN   (.lf 9)     1
                        2
                        3
                        .EN   (.lf 9)
This ruins the line numbers in groff and eqn diagnostics. eqn 1.22.4 does
it right.