util: New flag UNQUOTE_UNESCAPE_RELAX for unquote_first_word

The new flag UNQUOTE_UNESCAPE_RELAX preserves unrecognized escape
sequences verbatim in unquote_first_word, either when it's a trailing
backslash (similar to UNQUOTE_RELAX, but in this case keep the extra
backslash in the output) or in the middle of a sequence string.

Add unit test cases to ensure the new flag works as expected and to
prevent regressions from being introduced.

Tested with a follow up commit converting config_parse_exec() to start
using unquote_first_word, in which case this flags makes it possible to
preserve unrecognized escape sequences.

Relevant bug: https://bugs.freedesktop.org/show_bug.cgi?id=90794

src/basic/util.h

@@ -839,8 +839,9 @@ int is_dir(const char *path, bool follow);
 int is_device_node(const char *path);
 
 typedef enum UnquoteFlags {
-        UNQUOTE_RELAX     = 1,
-        UNQUOTE_CUNESCAPE = 2,
+        UNQUOTE_RELAX           = 1,
+        UNQUOTE_CUNESCAPE       = 2,
+        UNQUOTE_CUNESCAPE_RELAX = 4,
 } UnquoteFlags;
 
 int unquote_first_word(const char **p, char **ret, UnquoteFlags flags);