Retrieve URL asynchronously and call CALLBACK with CBARGS when finished.
URL is either a string or a parsed URL. If it is a string
containing characters that are not valid in a URI, those
characters are percent-encoded; see `url-encode-url'.
CALLBACK is called when the object has been completely retrieved, with
the current buffer containing the object, and any MIME headers associated
with it. It is called as (apply CALLBACK STATUS CBARGS).
STATUS is a plist representing what happened during the request,
with most recent events first, or an empty list if no events have
occurred. Each pair is one of:
(:redirect REDIRECTED-TO) - the request was redirected to this URL
(:error (ERROR-SYMBOL . DATA)) - an error occurred. The error can be
signaled with (signal ERROR-SYMBOL DATA).
Return the buffer URL will load into, or nil if the process has
already completed (i.e. URL was a mailto URL or similar; in this case
the callback is not called).
The variables `url-request-data', `url-request-method' and
`url-request-extra-headers' can be dynamically bound around the
request; dynamic binding of other variables doesn't necessarily
take effect.
If SILENT, then don't message progress reports and the like.
If INHIBIT-COOKIES, cookies will neither be stored nor sent to
the server.
If URL is a multibyte string, it will be encoded as utf-8 and
URL-encoded before it's used.
(fn URL CALLBACK &optional CBARGS SILENT INHIBIT-COOKIES)