util: add generic calls for prefixing a root directory to a path

So far a number of utilities implemented their own calls for this, unify
them in prefix_root() and prefix_roota(). The former uses heap memory,
the latter allocates from the stack via alloca().

Port over most users of a --root= logic.

src/tmpfiles/tmpfiles.c

@@ -1926,7 +1926,7 @@ static int parse_line(const char *fname, unsigned line, const char *buffer) {
         if (arg_root) {
                 char *p;
 
-                p = strappend(arg_root, i.path);
+                p = prefix_root(arg_root, i.path);
                 if (!p)
                         return log_oom();