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.

cmp.1

@@ -5,7 +5,7 @@
 .Nm cmp
 .Nd compare two files
 .Sh SYNOPSIS
-.Nm cmp
+.Nm
 .Op Fl l | Fl s
 .Ar file1 file2
 .Sh DESCRIPTION
@@ -14,7 +14,6 @@ compares two files byte by byte. If the files differ,
 .Nm
 prints the byte and
 line number at which the difference occurred.
-.Pp
 .Sh OPTIONS
 .Bl -tag -width Ds
 .It Fl l