.jshintrc

errors: introduce SecurityError and use it in a few places

This is part of
https://www.mercurial-scm.org/wiki/ErrorCategoriesPlan. There are
perhaps more errors in `sslutil.py` that should raise `SecurityError`;
I picked the most clear ones to start with.

Differential Revision: https://phab.mercurial-scm.org/D9390

{
    // Enforcing
    "eqeqeq"        : true,     // true: Require triple equals (===) for comparison
    "forin"         : true,     // true: Require filtering for..in loops with obj.hasOwnProperty()
    "freeze"        : true,     // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
    "nonbsp"        : true,     // true: Prohibit "non-breaking whitespace" characters.
    "undef"         : true,     // true: Require all non-global variables to be declared (prevents global leaks)

    // Environments
    "browser"       : true      // Web Browser (window, document, etc)
}