Type-check and lint the macOS build from Linux (closes #50)

script/lint-darwin (make lint-darwin; run by script/check, and its
commands by the Dockerfile lint stage) runs go vet and golangci-lint
with GOOS=darwin and cgo off. Compiling cgo for macOS needs Apple's SDK,
and keychainunlocker.go uses go-keychain, which is cgo there, so it and
its tests are now built only with cgo on macOS, like internal/macse;
their stubs serve a macOS build without cgo. checkMacOSAvailable moves
to seunlocker_darwin.go. The findings in the newly checked files are
fixed, and lines over 88 columns in the unchecked ones are wrapped.

Model: opus-5-5

script/check

@@ -1,7 +1,6 @@
 #!/bin/sh
-# script/check: run all checks (test, lint, fmt-check). Our own
-# extension to scripts-to-rule-them-all. Must not modify any files.
-# Generic: usually needs no adaptation.
+# script/check: run all checks (test, lint, lint-darwin, fmt-check). Our
+# own extension to scripts-to-rule-them-all. Must not modify any files.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -9,6 +8,7 @@ SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
 main() {
     "$SCRIPT_DIR/test"
     "$SCRIPT_DIR/lint"
+    "$SCRIPT_DIR/lint-darwin"
     "$SCRIPT_DIR/fmt-check"
 }