bug#20823: argv mangled by locale

[Zefram <zefram@fysh.org>]

When guile-2.0 stores argv for later access via program-arguments,
it sometimes decodes the underlying octet string according to the
nominal character encoding of the locale suggested by the environment.
This is a problem, because the arguments are not necessarily encoded
that way, and may not even be encodings of character strings at all.
The decoding is lossy, where the octet string isn't consistent with the
character encoding, so the original octet string cannot be recovered
from the mangled form.  I don't see any Scheme interface that reliably
retrieves the command line arguments without locale decoding.

The decoding doesn't follow the usual rules for locale control.  It is
not at all sensitive to setlocale, which is understandable due to the
arguments being acquired before any of the actual program's code runs.
Empirically, if the environment nominates no locale, "POSIX", or a
non-existent locale, then argv is decoded according to ISO-8859-1, thus
preserving the octets.  If the environment nominates an extant locale
other than "POSIX", then argv is decoded according to that locale's
nominal character encoding.

Demos:

$ env - guile-2.0 -c '(write (map char->integer (string->list (cadr (program-arguments))))) (newline)' $'L\xc3\xa9on'  
(76 195 169 111 110)
$ env - LANG=C guile-2.0 -c '(write (map char->integer (string->list (cadr (program-arguments))))) (newline)' $'L\xc3\xa9on' 
(76 63 63 111 110)
$ env - LANG=de_DE.utf8 guile-2.0 -c '(write (map char->integer (string->list (cadr (program-arguments))))) (newline)' $'L\xc3\xa9on' 
(76 233 111 110)
$ env - LANG=de_DE.iso88591 guile-2.0 -c '(write (map char->integer (string->list (cadr (program-arguments))))) (newline)' $'L\xc3\xa9on' 
(76 195 169 111 110)

The actual data passed between processes is an octet string, and
there really needs to be some reliable way to access that octet string.
My comments about resolution in bug#20822 "environment mangled by locale"
mostly apply here too, with a slight change: it seems necessary to store
the original octet strings and decode at the time program-arguments is
called.  With that change, the decoding can be responsive to setlocale
(and in particular can reliably use ISO-8859-1 in the absence of
setlocale).

-zefram