bug#33934: 26.1; server.el/emacsclient mismatch regarding socket...

[kris]

emacsclient has the -s option for specifying a UNIX socket directory
(emacslient -h says filename but it is a directory in the code - the bug
report!).
but server.el uses defvar to set server-socket-dir and there appears no
way to override it.

I changed defvar to defcustom to continue.
this is useful for my use case of running separate emacs
processes per project directory in order to do file change monitoring
with emacsclient -e signalling the correct session.

emacs forever!