basic/proc-cmdline: allow parsing of cmdline from a string

Comes with tests.

Also add direct test for $SYSTEMD_PROC_CMDLINE.

In test-proc-cmdline, "true" was masquerading as PROC_CMDLINE_STRIP_RD_PREFIX,
fix that. Also, reorder functions to match call order.

src/basic/proc-cmdline.c

@@ -39,17 +39,12 @@ int proc_cmdline(char **ret) {
                 return read_one_line_file("/proc/cmdline", ret);
 }
 
-int proc_cmdline_parse(proc_cmdline_parse_t parse_item, void *data, unsigned flags) {
-        _cleanup_free_ char *line = NULL;
+int proc_cmdline_parse_given(const char *line, proc_cmdline_parse_t parse_item, void *data, unsigned flags) {
         const char *p;
         int r;
 
         assert(parse_item);
 
-        r = proc_cmdline(&line);
-        if (r < 0)
-                return r;
-
         p = line;
         for (;;) {
                 _cleanup_free_ char *word = NULL;
@@ -85,6 +80,19 @@ int proc_cmdline_parse(proc_cmdline_parse_t parse_item, void *data, unsigned fla
         return 0;
 }
 
+int proc_cmdline_parse(proc_cmdline_parse_t parse_item, void *data, unsigned flags) {
+        _cleanup_free_ char *line = NULL;
+        int r;
+
+        assert(parse_item);
+
+        r = proc_cmdline(&line);
+        if (r < 0)
+                return r;
+
+        return proc_cmdline_parse_given(line, parse_item, data, flags);
+}
+
 static bool relaxed_equal_char(char a, char b) {
         return a == b ||
                 (a == '_' && b == '-') ||