Convert MOZILLA_OFFICIAL conditionals to MC_OFFICIAL where needed.

- `--enable-official-branding` implies `MC_OFFICIAL` (no need to specifically set it)
- `--enable-official-vendor` can be used to set `MC_OFFICIAL` on builds without `--enable-official-branding` that should still be considered official release versions.
- `MC_OFFICIAL` implies `--enable-release`, meaning `DEVELOPER_OPTIONS` isn't set
- `MC_OFFICIAL` makes `nsXULAppInfo.getIsOfficial` return `true`
- `MC_OFFICIAL` makes `AppConstants.MOZILLA_OFFICIAL` (for compatibility in extensions) and `AppConstants.MC_OFFICIAL` return `true`
- Optional, for the time being: `MOZILLA_OFFICIAL` is still present in some places in case someone wants to build a Mozilla-alike official application and has the rights and necessary keys to use Mozilla-official third-party services. This must always be combined with `MC_OFFICIAL` to have a sane combination of defines. This may be removed in the future.

build/autoconf/compiler-opts.m4

@@ -7,7 +7,7 @@ dnl Add compiler specific options
 AC_DEFUN([MOZ_DEFAULT_COMPILER],
 [
 dnl set DEVELOPER_OPTIONS early; MOZ_DEFAULT_COMPILER is usually the first non-setup directive
-  if test -z "$MOZILLA_OFFICIAL"; then
+  if test -z "$MC_OFFICIAL"; then
     DEVELOPER_OPTIONS=1
   fi
   MOZ_ARG_ENABLE_BOOL(release,