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/system.scm

@@ -1141,23 +1141,13 @@ for i in /etc/profile.d/*.sh; do
 done
 unset i
 
-if [ -n \"$BASH_VERSION\" -a -f /etc/bashrc ]
-then
-  # Load Bash-specific initialization code.
-  . /etc/bashrc
+if [ -n \"$BASH_VERSION\" -a -f /etc/bashrc ]; then
+  # Load Bash-specific initialization code, taking care to not source
+  # /etc/bashrc when invoked from a non-interactive SSH shell,
+  # to avoid recursion (/etc/bashrc also sources /etc/profile
+  # in the non-login, non-interactive SSH case).
+  [[ $- != *i* && -n $SSH_CLIENT ]] || source /etc/bashrc
 fi
-"))
-
-        (bashrc    (plain-file "bashrc" "\
-# Bash-specific initialization.
-
-# Provide a default prompt.  The user's ~/.bashrc can override it.
-PS1='\\u@\\h \\w${GUIX_ENVIRONMENT:+ [env]}\\$ '
-
-for i in /etc/bashrc.d/*.sh; do
-    [[ -r $i ]] && source \"$i\"
-done
-unset i
 ")))
     (service etc-service-type
      `(("os-release" ,os-release)
@@ -1168,7 +1158,7 @@ unset i
        ("issue" ,issue)
        ,@(if nsswitch `(("nsswitch.conf" ,nsswitch)) '())
        ("profile" ,profile)
-       ("bashrc" ,bashrc)
+       ("bashrc" ,%default-bashrc)
        ;; Write the operating-system-host-name to /etc/hostname to prevent
        ;; NetworkManager from changing the system's hostname when connecting
        ;; to certain networks.  Some discussion at