Add support in emacsclient for EMACS_SOCKET_NAME

If the '--socket-name' argument is unspecified, the environment
variable 'EMACS_SOCKET_NAME' is now consulted with the same
semantics.  This mirrors the behavior of the '--server-file' argument,
and allows for easier configuration of emacsclient when the socket is
in a location other than 'TMPDIR' or '/tmp'.

* emacsclient.c (set_socket): Add support for the
EMACS_SOCKET_NAME environment variable.  (Bug#33095)

* misc.texi (emacsclient Options):
* emacsclient.1: Document the EMACS_SOCKET_NAME environment
variable.

* etc/NEWS: Announce the new feature.

Copyright-paperwork-exempt: yes

doc/emacs/misc.texi

@@ -1966,6 +1966,10 @@ is given by the variable @code{server-name} on the Emacs server.  If
 this option is omitted, @command{emacsclient} connects to the first
 server it finds.  (This option is not supported on MS-Windows.)
 
+Alternatively, you can set the @env{EMACS_SOCKET_NAME} environment
+variable to point to the server socket.  (The command-line option
+overrides the environment variable.)
+
 @item -t
 @itemx --tty
 @itemx -nw

doc/man/emacsclient.1

@@ -94,6 +94,7 @@ open a new Emacs frame on the current terminal
 .TP
 .B \-s, \-\-socket-name=FILENAME
 use socket named FILENAME for communication.
+This can also be specified via the EMACS_SOCKET_NAME environment variable.
 .TP
 .B \-V, \-\-version
 print version information and exit

etc/NEWS

@@ -119,6 +119,12 @@ the new version of the file again.)
 
 * Changes in Emacs 27.1
 
++++
+** emacsclient uses EMACS_SOCKET_NAME if --socket-name is not set.
+The behavior is identical to the EMACS_SERVER_FILE, in that the
+command line value will override the environment, and the natural
+default to TMPDIR, then /tmp, continues to apply.
+
 +++
 ** The function 'read-passwd' uses '*' as default character to hide passwords.
 

lib-src/emacsclient.c

@@ -1409,6 +1409,9 @@ set_socket (int no_exit_if_error)
 
 #ifndef NO_SOCKETS_IN_FILE_SYSTEM
   /* Explicit --socket-name argument.  */
+  if (!socket_name)
+    socket_name = egetenv ("EMACS_SOCKET_NAME");
+
   if (socket_name)
     {
       s = set_local_socket (socket_name);