Fix formatting of tables.

Fix formatting of tables.

Specifically, use .Ta instead of tabs to separate column entries.  While
here fix a few other things:
- Use .Sy for all column headers (previously only the first column header
  was bold)
- Use .Dv to markup constants used for MIB names.
- Use "1234" and "4321" for the byte order descriptions without
  thousands separators.
- Mark up header files in the first table with .In.

MFC after:	2 weeks