Add --r6rs command-line option

* doc/ref/guile-invoke.texi (Command-line Options): Document --r6rs.
* doc/ref/r6rs.texi (R6RS Incompatibilities): Mention that --r6rs calls
  install-r6rs!.
* module/ice-9/command-line.scm (*usage*, compile-shell-switches):
  Implement --r6rs.

module/ice-9/command-line.scm

@@ -138,6 +138,8 @@ If FILE begins with `-' the -s switch is mandatory.
   -q             inhibit loading of user init file
   --use-srfi=LS  load SRFI modules for the SRFIs in LS,
                  which is a list of numbers like \"2,13,14\"
+  --r6rs         change initial Guile environment to better support
+                 R6RS
   -h, --help     display this help and exit
   -v, --version  display version information and exit
   \\              read arguments from following script lines"))
@@ -370,6 +372,10 @@ If FILE begins with `-' the -s switch is mandatory.
               (parse args
                      (cons `(use-srfis ',srfis) out))))
 
+           ((string=? "--r6rs" arg)
+            (parse args
+                   (cons '(install-r6rs!) out)))
+
            ((string=? arg "--listen")   ; start a repl server
             (parse args
                    (cons '((@@ (system repl server) spawn-server)) out)))