Added support for FTPS

* doc/wget.texi: updated documentation to reflect the new FTPS functionality.
 * src/ftp-basic.c (ftp_greeting): new function to read the server's greeting.
   (ftp_login): greeting code was previously here. Moved to ftp_greeting to
   support FTPS implicit mode.
   (ftp_auth): wrapper around the AUTH TLS command.
   (ftp_ccc): wrapper around the CCC command.
   (ftp_pbsz): wrapper around the PBSZ command.
   (ftp_prot): wraooer around the PROT command.
 * src/ftp.c (get_ftp_greeting): new static function.
   (init_control_ssl_connection): new static function to start SSL/TLS on the
   control channel.
   (getftp): added hooks to support FTPS commands (RFCs 2228 and 4217).
   (ftp_loop_internal): test for new FTPS error codes.
 * src/ftp.h: new enum 'prot_level' with available FTPS protection levels +
   prototypes of previous functions. New flag for enum 'wget_ftp_fstatus' to track
   whether the data channel has some security mechanism enabled or not.
 * src/gnutls.c (struct wgnutls_transport_context): new field 'session_data'.
   (wgnutls_close): free GnuTLS session data before exiting.
   (ssl_connect_wget): save/resume SSL/TLS session.
 * src/http.c (establish_connection): refactor ssl_connect_wget call.
   (metalink_from_http): take into account SCHEME_FTPS as well.
 * src/init.c, src/main.c, src/options.h: new command line/wgetrc options.
   (main): in recursive downloads, check for SCHEME_FTPS as well.
 * src/openssl.c (struct openssl_transport_context): new field 'sess'.
   (ssl_connect_wget): save/resume SSL/TLS session.
 * src/retr.c (retrieve_url): check new scheme SCHEME_FTPS.
 * src/ssl.h (ssl_connect_wget): refactor. New parameter of type 'int *'.
 * src/url.c. src/url.h: new scheme SCHEME_FTPS.
 * src/wget.h: new FTPS error codes.
 * src/metalink.h: support FTPS scheme.

doc/wget.texi

@@ -2008,6 +2008,43 @@ this option has no effect.  Symbolic links are always traversed in this
 case.
 @end table
 
+@section FTPS Options
+
+@table @samp
+@item --ftps-implicit
+This option tells Wget to use FTPS implicitly. Implicit FTPS consists of initializing
+SSL/TLS from the very beginning of the control connection. This option does not send
+an @code{AUTH TLS} command: it assumes the server speaks FTPS and directly starts an
+SSL/TLS connection. If the attempt is successful, the session continues just like
+regular FTPS (@code{PBSZ} and @code{PROT} are sent, etc.).
+Implicit FTPS is no longer a requirement for FTPS implementations, and thus
+many servers may not support it. If @samp{--ftps-implicit} is passed and no explicit
+port number specified, the default port for implicit FTPS, 990, will be used, instead
+of the default port for the "normal" (explicit) FTPS which is the same as that of FTP,
+21.
+
+@item --no-ftps-resume-ssl
+Do not resume the SSL/TLS session in the data channel. When starting a data connection,
+Wget tries to resume the SSL/TLS session previously started in the control connection.
+SSL/TLS session resumption avoids performing an entirely new handshake by reusing
+the SSL/TLS parameters of a previous session. Typically, the FTPS servers want it that way,
+so Wget does this by default. Under rare circumstances however, one might want to
+start an entirely new SSL/TLS session in every data connection.
+This is what @samp{--no-ftps-resume-ssl} is for.
+
+@item --ftps-clear-data-connection
+All the data connections will be in plain text. Only the control connection will be
+under SSL/TLS. Wget will send a @code{PROT C} command to achieve this, which must be
+approved by the server.
+
+@item --ftps-fallback-to-ftp
+Fall back to FTP if FTPS is not supported by the target server. For security reasons,
+this option is not asserted by default. The default behaviour is to exit with an error.
+If a server does not successfully reply to the initial @code{AUTH TLS} command, or in the
+case of implicit FTPS, if the initial SSL/TLS connection attempt is rejected, it is
+considered that such server does not support FTPS.
+@end table
+
 @node Recursive Retrieval Options, Recursive Accept/Reject Options, FTP Options, Invoking
 @section Recursive Retrieval Options