Default to sRGB colors for CSI RGB values.

- By default consider CSI 24-bit RGB values to be display independent
  sRGB colors.
- Add an advanced option to disable this and consider the values to
  be calibrated to the given display that iTerm2 is running on.

Defaulting to interpret received CSI RGB values as sRGB makes more sense than the current method (that they are pre-calibrated to the display). Normally one is using iTerm to connect to a variety of hosts (also perhaps from a variety of hosts e.g., laptop, desktop, etc). These hosts want to send colors as sRGB and have them calibrated by the local host to the local display, rather than have to have a custom set of colors for each and every terminal (and their displays) that may connect to them.

I think it makes so much sense to use sRGB here that I'm not sure we even need a preference option or a choice, but since I'm changing the (as yet unreleased) behavior I thought I'd put it in there anyway.
I don't see why it's necessary to have a different key-space for sRGB colors. Why not use the user preference to switch all 24-bit colors between sRGB and calibrated? I think that would simplify this code a lot.