path-util: get rid of prefix_root()

prefix_root() is equivalent to path_join() in almost all ways, hence
let's remove it.

There are subtle differences though: prefix_root() will try shorten
multiple "/" before and after the prefix. path_join() doesn't do that.
This means prefix_root() might return a string shorter than both its
inputs combined, while path_join() never does that. I like the
path_join() semantics better, hence I think dropping prefix_root() is
totally OK. In the end the strings generated by both functon should
always be identical in terms of path_equal() if not streq().

This leaves prefix_roota() in place. Ideally we'd have path_joina(), but
I don't think we can reasonably implement that as a macro. or maybe we
can? (if so, sounds like something for a later PR)

Also add in a few missing OOM checks

src/cryptsetup/cryptsetup-generator.c

@@ -235,7 +235,7 @@ static int create_disk(
                 if (r < 0)
                         return log_error_errno(r, "Failed to generate keydev mount unit: %m");
 
-                p = prefix_root(keydev_mount, password_escaped);
+                p = path_join(keydev_mount, password_escaped);
                 if (!p)
                         return log_oom();