config.nmake: Do some minor changes (mostly in comments).

- Update comment: /O2 enables /Gs which is "Control Stack Checking Calls" not "Buffer Security Check"
- Remove explicit setting of /GS (Buffer Security Check) since it's set by default.
  (The explicit setting did not apply to the CFLAGS used to compile Wireshark
   files because the setting was in the wrong place, but, fortunately, /GS is on
   by default.
- Add a comment about /GS being on by default.
- Remove some trailing whitespace.

Change-Id: I6cf631ee1ba0fb8556171861c7e5eb6d959c8f0c
Reviewed-on: https://code.wireshark.org/review/3228
Reviewed-by: Anders Broman <a.broman58@gmail.com>