bug#19594: url-ftp: should not be aliased to url-file; url-file: use Tramp

[Ivan Shmakov]

Package:  emacs
Severity: minor

        As of 88bc8332eb14 (2015-01-07 13:51:41 +0000), url-ftp is
        implemented as an alias to url-file, which results in the
        following (unexpected) behavior.

(let ((url-proxy-services nil))
  (url-retrieve-synchronously "ftp://localhost/dev/null";))

        Returns: a buffer.  Expected: failure, as there’s no FTP server
        running on localhost.

(let ((url-proxy-services nil))
  (url-retrieve-synchronously "file://remote.example/dev/null"))

        Results in a internal-ange-ftp-mode buffer opened.  Expected:
        the contents of the file, as retrieved via Tramp.

        I understand that FTP was once the only widespread file transfer
        protocol /and/ that ange-ftp was once the only Emacs package
        providing access to remote filesystems.  Now that Emacs has
        Tramp, I see no reason for using ange-ftp for retrieving
        resources denoted by file: scheme URIs, – unless so is the
        preference of the user.

        My suggestion would be that the FTP-specific code be split off
        url-file.el into url-ftp.el (so that ftp: URIs would exclusively
        denote resources available via FTP), /and/ new Tramp interface
        (hopefully trivial) be coded into url-file.el.

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