add some mandoc pages and fix some warnings

- add .Os, it is mandatory.
- don't redeclare .Nm when it's not needed.
- fix some warnings (checked with mandoc -Tlint).
- remove some leftover old stuff.

nl.1

@@ -5,7 +5,7 @@
 .Nm nl
 .Nd line numbering filter
 .Sh SYNOPSIS
-.Nm nl
+.Nm
 .Op Fl b Ar type
 .Op Fl i Ar incr
 .Op Fl s Ar sep
@@ -33,8 +33,7 @@ Only non-empty lines (default).
 Only lines which match
 .Ar expr ,
 a regular expression as defined in
-.IR regex (7).
-.TP
+.Xr regex 7
 .El
 .It Fl i Ar incr
 Defines the increment between numbered lines.