win32 readline

Readline is pretty badly broken under mingw. Basically, it disables the
alt-gr key, which renders psql almost useless on most locales (no way to
type backslash, and a whole lot of other characters, for example).

This patch disables readline on win32. (meaning it's back to working the
way it did in <= 7.4, since msvc compiles have never supported readline)

The patch also contains two definitions for win32/port/security.c that
were for some reason not included when the rest of the win32-admin-check
patch was applied earlier, that I stumbled on while making this patch.

//Magnus