condition: split out order operator enum

Let's move the operator enum into its own .c/.h file, so that we can
reuse it elsewhere, in particular systemd-analyze's compare-versions
logic.

Let's rename the concept CompareOperator, since it is nowadays
genericlaly about both order *and* fnmatch comparisons, hence just
naming it "order" is misleading.

src/shared/compare-operator.c

@@ -0,0 +1,38 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "compare-operator.h"
+#include "string-util.h"
+
+CompareOperator parse_compare_operator(const char **s, bool allow_fnmatch) {
+        static const char *const prefix[_COMPARE_OPERATOR_MAX] = {
+                [COMPARE_FNMATCH_EQUAL] = "=$",
+                [COMPARE_FNMATCH_UNEQUAL] = "!=$",
+
+                [COMPARE_LOWER_OR_EQUAL] = "<=",
+                [COMPARE_GREATER_OR_EQUAL] = ">=",
+                [COMPARE_LOWER] = "<",
+                [COMPARE_GREATER] = ">",
+                [COMPARE_EQUAL] = "=",
+                [COMPARE_UNEQUAL] = "!=",
+        };
+
+        assert(s);
+
+        if (!*s) /* Hmm, we already reached the end, for example because extract_first_word() and
+                  * parse_compare_operator() are use on the same string? */
+                return _COMPARE_OPERATOR_INVALID;
+
+        for (CompareOperator i = 0; i < _COMPARE_OPERATOR_MAX; i++) {
+                const char *e;
+
+                e = startswith(*s, prefix[i]);
+                if (e) {
+                        if (!allow_fnmatch && COMPARE_OPERATOR_IS_FNMATCH(i))
+                                break;
+                        *s = e;
+                        return i;
+                }
+        }
+
+        return _COMPARE_OPERATOR_INVALID;
+}