testsuite batch94: declare/unalias/exec/hash/unset diagnostics + ${a[@]@A}

[brianjfox]

Batch 94 of bash 5.3 test-suite conformance work, driven by the errors test (2nd-largest byte-diff after intl). Five independent conformance fixes:

declare/typeset/local diagnostics — reject non-identifier operands (declare /bin/sh, a -z left after --) with not a valid identifier; reject -f/-F with an assignment (*cannot use -f' to make functions*); prefix a readonly-assignment failure with the builtin name and propagate its exit status; refuse +rclearing an existing readonly.local -` stays exempt as bash's save-options operand.

unalias / exec usage — bare unalias (no name) and exec with a bad option now print their usage lines.

set +o hashall — replaced the no-op with a real opt_hashall state wired through set -h/+h, set -o hashall, the -h invocation flag, $SHELLOPTS and $-. hash now fails with hashing disabled when off; $- drops h.

unset bad subscript — unset a[-2] on an empty/too-short indexed array now reports unset: [SUB]: bad array subscript (exit 1) instead of silently no-op'ing.

${a[@]@A} — the whole-array @A transform applied the per-element scalar operator (garbage declare -A a=<value> per element). Extracted declare -p's formatting into declare_var_string and reused it, so ${a[@]@A} recreates the array and eval "${a[@]@A}" round-trips byte for byte.

Scoreboard impact (byte-diffs vs bash 5.3)

test	before	after
errors	297	272
nameref	256	247
exp	44	40
assoc	88	86

No regressions in any other test. ctest 22/22 and run_diff all 234 scripts match bash.