erc: Theme erc-startup-file-list

erc-startup-file-list is a list of .bashrc-like files that are
searched for by erc, which executes the first extant and readable one.

If the executed file ends with a ".el" extension, then it is loaded as
an Elisp script, while if it does not end with that extension, then it
is treated as a bunch of IRC commands.

Following the upstream convention, we put the Elisp variant prior to
the non-Elisp variant in the list.

no-littering.el

@@ -130,6 +130,7 @@ This variable has to be set before `no-littering' is loaded.")
     (setq ede-project-placeholder-cache-file (var "ede-projects.el"))
     (setq erc-dcc-get-default-directory    (var "erc/dcc/"))
     (setq erc-log-channels-directory       (var "erc/log-channels/"))
+    (setq erc-startup-file-list            (list (etc "erc/startup.el") (etc "erc/startup") ".ercrc.el" ".ercrc"))
     (setq eshell-aliases-file              (etc "eshell/aliases"))
     (setq eshell-directory-name            (var "eshell/"))
     (setq eshell-login-script              (etc "eshell/login"))