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.

doc/ref/guile-invoke.texi

@@ -1,7 +1,7 @@
 @c -*-texinfo-*-
 @c This is part of the GNU Guile Reference Manual.
-@c Copyright (C)  1996, 1997, 2000-2005, 2010, 2011, 2013, 2014,
-@c   2016 Free Software Foundation, Inc.
+@c Copyright (C) 1996-1997,2000-2005,2010-2011,2013-2014,2016,2019
+@c   Free Software Foundation, Inc.
 @c See the file guile.texi for copying conditions.
 
 @node Invoking Guile
@@ -137,6 +137,11 @@ Here is an example that loads the modules SRFI-8 ('receive') and SRFI-13
 guile --use-srfi=8,13
 @end example
 
+@item --r6rs
+@cindex r6rs (command line)
+Adapt Guile's initial environment to better support R6RS.  @xref{R6RS
+Incompatibilities}, for some caveats.
+
 @item --debug
 @cindex debugging virtual machine (command line)
 Start with the debugging virtual machine (VM) engine.  Using the

doc/ref/r6rs.texi

@@ -119,6 +119,10 @@ conventions.  @xref{Load Paths}.  Also, enable R6RS unicode escapes in
 strings; see the discussion above.
 @end deffn
 
+Finally, note that the @code{--r6rs} command-line argument will call
+@code{install-r6rs!} before calling user code.  R6RS users probably want
+to pass this argument to their Guile.
+
 @node R6RS Standard Libraries
 @subsection R6RS Standard Libraries
 

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)))