Stack cookie check + edge case

[woodruffw]

We should also probably check whether the program was built with /GS, which inserts stack cookies.

Like /DYNAMICBASE, /GS has an interesting edge case: if the user defines a custom entry point via /ENTRY and forgets to call __security_init_cookie() within it, then the cookie value is set to a default value that makes circumvention much easier.

[woodruffw]

Based on https://reverseengineering.stackexchange.com/a/6709, the right approach is:

1. First, check IMAGE_LOAD_CONFIG_DIRECTORY.SecurityCookie. If that's nonzero, then the cookie is at that VA and GS is enabled.
2. If SecurityCookie is zero, then scan for either the default cookie (0xBB40E64E) or for one of the characteristic signatures for __security_check_cookie.

Just doing step 1 is probably sufficient for our purposes -- it looks like a false negatives with zeroed SecurityCookies have been seen with older binaries, but not with anything that SL2 is targeting.