core: interpret \; token in ExecStart as escaped ;

Some commands (like 'find') take a semicolon as separate arg. With
current parser implementation there is no way to pass one.

Patch adds token \;

src/core/load-fragment.c

@@ -483,6 +483,8 @@ int config_parse_exec(
                 FOREACH_WORD_QUOTED(w, l, rvalue, state) {
                         if (strncmp(w, ";", MAX(l, 1U)) == 0)
                                 break;
+                        else if (strncmp(w, "\\;", MAX(l, 1U)) == 0)
+                                w ++;
 
                         if (honour_argv0 && w == rvalue) {
                                 assert(!path);