MFC r315132,r315133,r315186:

r315132:

Use .Dv when referencing NULL

This is the correct markup macro, as opposed to .Va (variable names)

While here, annotate several bare references to NULL with .Dv.

r315133:

lib/libcam/cam.3: fix manpage warnings

• spelling: "mis-named" should be "misnamed".
• delete spaces interspersed in literal representation of struct cam_device as hard-tabs separate the types and fields.
• Add commas after e.g..

r315186:

lib/libcam/cam.3: note that cam_freeccb(3) with ccb == NULL is a no-op

This allows me to accurately test this scenario, and for others to rely
on the behavior, instead of relying on knowledge obtained via code
inspection.

Wording borrowed from free(3).

Requested by: ken (D9928)