system: Factorize bashrc default configuration.

This factorizes out the remaining bashrc bits from /etc/skel/.bashrc to a the
template used for both /etc/bashrc on Guix System and ~/.bashrc for
home-bash-service-type.

Rationale: The use of /etc/skel introduce state: the file is only copied
originally when the user account is created, and never (automatically)
refreshed again.

* gnu/system.scm (operating-system-etc-service):
<profile>: Guard against souring /etc/bashrc in non-interactive, SSH case.
<bashrc>: Use %default-bashrc, having migrated the remaining definitions to...
* gnu/system/shadow.scm (%default-bashrc): ... here.  Factorize aliases to...
* gnu/services.scm (%default-bash-aliases): ... here.
(%default-bashrc-d-aliases): New variable.
(%default-etc-bashrc-d-files): Include it in the default configuration.
* gnu/services/base.scm (%base-services): Register etc-bashrc-d-service-type.
* gnu/home/services/shells.scm (add-bash-configuration): Do not set PS1, now
part of %default-bashrc.
(home-bash-configuration) [guix-defaults?]: Update doc.
[aliases]: Set %default-bash-aliases as the default value.  Update doc.
* doc/guix.texi (Shells Home Services): Update documentation.
(Service Reference): Update example.

Change-Id: I340c614983a78fd20a9c4a9705e7fc542ae9b513

gnu/services/base.scm

@@ -4138,7 +4138,10 @@ login manager daemon.")
         (service mingetty-service-type (mingetty-configuration
                                          (tty "tty5")))
         (service mingetty-service-type (mingetty-configuration
-                                         (tty "tty6")))
+                                        (tty "tty6")))
+
+        ;; Extra Bash configuration including Bash completion and aliases.
+        (service etc-bashrc-d-service-type)
 
         (service static-networking-service-type
                  (list %loopback-static-networking))