Function: open-protocol-stream

Open a TCP connection to HOST, optionally with encryption.
Normally, return a network process object; with a non-nil
:return-list parameter, return a list instead (see below).
Input and output work as for subprocesses; `delete-process'
closes it.

NAME is the name for the process. It is modified if necessary to
make it unique.
BUFFER is a buffer or buffer name to associate with the process.
Process output goes at end of that buffer. BUFFER may be nil,
meaning that the process is not associated with any buffer.
HOST is the name or IP address of the host to connect to.
SERVICE is the name of the service desired, or an integer specifying
a port number to connect to.

The remaining PARAMETERS should be a sequence of keywords and

:type specifies the connection type, one of the following:
nil or `network'
-- Begin with an ordinary network connection, and if
the parameters :success and :capability-command
are also supplied, try to upgrade to an encrypted
connection via STARTTLS. Even if that
fails (e.g. if HOST does not support TLS), retain
an unencrypted connection.
`plain' -- An ordinary, unencrypted network connection.
`starttls' -- Begin with an ordinary connection, and try
upgrading via STARTTLS. If that fails for any
reason, drop the connection; in that case the
returned object is a killed process.
`tls' -- A TLS connection.
`ssl' -- Equivalent to `tls'.
`shell' -- A shell connection.

:return-list specifies this function's return value.
If omitted or nil, return a process object. A non-nil means to
return (PROC . PROPS), where PROC is a process object and PROPS
is a plist of connection properties, with these keywords:
:greeting -- the greeting returned by HOST (a string), or nil.
:capabilities -- a string representing HOST's capabilities,
or nil if none could be found.
:type -- the resulting connection type; `plain' (unencrypted)
or `tls' (TLS-encrypted).

:end-of-command specifies a regexp matching the end of a command.

:end-of-capability specifies a regexp matching the end of the
response to the command specified for :capability-command.
It defaults to the regexp specified for :end-of-command.

:success specifies a regexp matching a message indicating a
successful STARTTLS negotiation. For instance, the default
should be "^3" for an NNTP connection.

:capability-command specifies a command used to query the HOST
for its capabilities. For instance, for IMAP this should be

:starttls-function specifies a function for handling STARTTLS.
This function should take one parameter, the response to the
capability command, and should return the command to switch on
STARTTLS if the server supports STARTTLS, and nil otherwise.

:always-query-capabilities says whether to query the server for
capabilities, even if we're doing a `plain' network connection.

:client-certificate should either be a list where the first
element is the certificate key file name, and the second
element is the certificate file name itself, or `t', which
means that `auth-source' will be queried for the key and the
certificate. This parameter will only be used when doing TLS
or STARTTLS connections. :use-starttls-if-possible is a boolean that says to do opportunistic STARTTLS upgrades even if Emacs doesn't have built-in TLS functionality. :nowait is a boolean that says the connection should be made asynchronously, if possible. (fn NAME BUFFER HOST SERVICE &rest PARAMETERS)