Drop -m/-u/-c short options for --mount/--umount/--cleanup

Now that -r/--run and -x/--exec cover normal use, --mount/--umount/--cleanup
are debug-only escape hatches not worth a short letter. Reassigned their
long_options codes to long-option-only constants (options::mount/umount/
cleanup) and dropped m:/u:/c: from getopt_long's own short-options string.

Updated the fixture smoke test (tests/run_test.py) and docs, which invoked
-m/-u/-c directly.

tests/run_test.py

@@ -15,7 +15,7 @@ def run(slocker_lite: str, *args: str) -> subprocess.CompletedProcess:
 def main() -> int:
     slocker_lite, fixture_tar = sys.argv[1], sys.argv[2]
 
-    mount = run(slocker_lite, "-m", fixture_tar)
+    mount = run(slocker_lite, "--mount", fixture_tar)
     if mount.returncode != 0:
         print(mount.stderr, file=sys.stderr)
         return mount.returncode
@@ -26,7 +26,7 @@ def main() -> int:
         return 1
     layer_id = match.group(1)
 
-    for args in (("-u", layer_id), ("-c", layer_id)):
+    for args in (("--umount", layer_id), ("--cleanup", layer_id)):
         result = run(slocker_lite, *args)
         if result.returncode != 0:
             print(result.stderr, file=sys.stderr)

tests/unit/test_cli_args.cpp

@@ -56,13 +56,19 @@ ParseResult run_parse(const std::vector<std::string>& args) {
 
 }  // namespace
 
-TEST_CASE("parse_args: -m <image.tar> selects Mode::mount", "[unit]") {
-    auto result = run_parse({"-m", "image.tar"});
+TEST_CASE("parse_args: --mount <image.tar> selects Mode::mount", "[unit]") {
+    auto result = run_parse({"--mount", "image.tar"});
     REQUIRE_FALSE(result.exit_code.has_value());
     CHECK(result.args.mode == Mode::mount);
     CHECK(result.args.mode_arg == "image.tar");
 }
 
+TEST_CASE("parse_args: -m is no longer a recognized short option", "[unit]") {
+    auto result = run_parse({"-m", "image.tar"});
+    REQUIRE(result.exit_code.has_value());
+    CHECK(*result.exit_code == 1);
+}
+
 TEST_CASE("parse_args: -h returns exit code 0 immediately", "[unit]") {
     auto result = run_parse({"-h"});
     REQUIRE(result.exit_code.has_value());
@@ -82,7 +88,7 @@ TEST_CASE("parse_args: no mode at all is a parse error", "[unit]") {
 }
 
 TEST_CASE("parse_args: two actions on the same command line is a parse error", "[unit]") {
-    auto result = run_parse({"-m", "a.tar", "-l", "dir"});
+    auto result = run_parse({"--mount", "a.tar", "-l", "dir"});
     REQUIRE(result.exit_code.has_value());
     CHECK(*result.exit_code == 1);
 }