Conform redirections: ambiguous redirect + noclobber (batch80)

[brianjfox]

Summary

Two coherent redirection-conformance fixes surfaced by redir.tests (byte-diff 186 → 178, no regressions elsewhere).

1. Ambiguous redirect (closes #277)

A redirection target is expanded (brace, parameter/command/arithmetic substitution, word splitting, pathname expansion) and bash requires the result to be exactly one word; zero or multiple words is an ambiguous redirect. gnash used expand_no_split (joining fields into one literal word), so z="a b"; cat < $z opened a file literally named a b.

New expand_redir_target runs the full pipeline and errors with the original unexpanded word text when the result isn't a single word. Applied to every filename redirection (<, >, >|, >>, <>, &>, &>>, {var}<file) and to the fd-dup operators (<&word / >&word).

$ z="a b"; cat < $z          → gnash: line 1: $z: ambiguous redirect
$ cat < $undefined            → gnash: line 1: $undefined: ambiguous redirect
$ echo hi > {a,b}             → gnash: line 1: {a,b}: ambiguous redirect
$ exec 3<&$unset             → gnash: line 1: $unset: ambiguous redirect

2. noclobber

set -o noclobber / set -C / -C was recognized but had no effect. Wire opt_noclobber through set/set -o, $-, and $SHELLOPTS, and honor it when opening a truncating output redirection.

open_redir_output mirrors bash's noclobber_open: a clobbering redirect (>/&>, not the overriding >|) refuses to overwrite an existing regular file (cannot overwrite existing file), creates a missing file exclusively, and writes to a non-regular file (device, fifo, /dev/null) without truncating. >>, &>>, and <> are unaffected.

Testing

• ctest 22/22
• Scoreboard: redir 186 → 178, zero regressions across the full test suite
• Verified each case byte-for-byte against source-built bash 5.3