shared/install: Preserve escape characters for escaped unit names

Since switching to extract_first_word with no flags for parsing
unit names in 4c9565eea5, escape
characters will be stripped from escaped unit names such as
"mnt-persistent\x2dvolume.mount" resulting in the unit not being
configured as defined.  Preserve escape characters again for
compatibility with existing preset definitions.

src/shared/install.c

@@ -2797,7 +2797,7 @@ static int split_pattern_into_name_and_instances(const char *pattern, char **out
         assert(out_instances);
         assert(out_unit_name);
 
-        r = extract_first_word(&pattern, &unit_name, NULL, 0);
+        r = extract_first_word(&pattern, &unit_name, NULL, EXTRACT_RETAIN_ESCAPE);
         if (r < 0)
                 return r;