fix(url-guard): restore the SSRF guard on bash 3.2

`${1,,}` is bash 4.0+. macOS ships bash 3.2 as /bin/bash, where it raises
"bad substitution"; the subshell then exited 1 — read as "not local" — so
`url-guard.sh host` returned rc 0 for localhost, 127.x, 10.x, 192.168.x,
172.16-31.x, 169.254.169.254 and metadata.google.internal. The guard failed
OPEN on every Mac, and url-guard.test.sh recorded it as 13 "got[0] want[2]".

`shopt -s nocasematch` (bash 3.1+) keeps both the ASCII-only folding that
LC_ALL=C gives and the no-fork property the lowercase expansion had.

Verified on /bin/bash 3.2: the ten local/private/metadata targets now return
rc 2 (case-folded variants included), legitimate hosts still rc 0.

lib/url-guard.sh

@@ -41,9 +41,14 @@ _rest_charset_ok() ( LC_ALL=C; case "$1" in
 # Literal local/private/metadata targets. This is a LITERAL check, not a DNS
 # one: it stops the obvious, not a hostname that resolves inward.
 _host_is_local() ( LC_ALL=C
-  # ${1,,} not tr: no fork, and no SC2018/SC2019 noise. Safe because the
-  # charset guard has already run — the string is [A-Za-z0-9.-] by here.
-  case "${1,,}" in
+  # `nocasematch` not ${1,,}: the lowercase expansion is bash 4.0+, and macOS
+  # ships bash 3.2 as /bin/bash — there it raised "bad substitution" and the
+  # subshell exited 1, i.e. "not local", so EVERY local/private/metadata host
+  # was allowed through. Keeps the no-fork property the lowercase form had.
+  # Safe because the charset guard has already run — the string is
+  # [A-Za-z0-9.-] by here, and LC_ALL=C keeps the folding ASCII-only.
+  shopt -s nocasematch
+  case "$1" in
     localhost|*.localhost|*.local|0.0.0.0|broadcasthost) exit 0 ;;
     127.*|10.*|169.254.*|192.168.*) exit 0 ;;
     172.1[6-9].*|172.2[0-9].*|172.3[01].*) exit 0 ;;