Clean up netmap(4) slightly

• Add missing sections for .Xr references.
• Replace .br with .Pp (the former macro is deprecated).
• Use the .Sx (section reference) macro when referring to LIBRARIES, not the .Xr (cross-reference) macro.
• Add commas after "e.g." and "i.e." [*].

Bump .Dd for the change

Approved by: luigi
MFC after: 1 week
Reported by: igor [*], make manlint
Sponsored by: Dell EMC Isilon
Differential Revision: D9859