system: Set ‘HISTSIZE’ in default bashrc.

This provides more appropriate defaults for today.  This change affects
/etc/bashrc, /etc/skel/.bashrc, along with ~/.bashrc for Home users.

* gnu/system/shadow.scm (%default-bashrc): Set ‘HISTSIZE’.

Change-Id: I4ffa7ad75c6f39f3f36d5a15ce420eae52e241e5

gnu/system/shadow.scm

@@ -172,6 +172,9 @@ for i in /etc/bashrc.d/*.sh; do
     [[ -r $i ]] && source \"$i\"
 done
 unset i
+
+# Increase the history size (default is 500 entries).
+HISTSIZE=10000
 "))
 
 (define %default-bash-profile