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/basic/path-util.h

@@ -116,10 +116,8 @@ int mkfs_exists(const char *fstype);
              _slash && ((*_slash = 0), true);                           \
              _slash = strrchr((prefix), '/'))
 
-char *prefix_root(const char *root, const char *path);
-
-/* Similar to prefix_root(), but returns an alloca() buffer, or
- * possibly a const pointer into the path parameter */
+/* Similar to path_join(), but only works for two components, and only the first one may be NULL and returns
+ * an alloca() buffer, or possibly a const pointer into the path parameter */
 #define prefix_roota(root, path)                                        \
         ({                                                              \
                 const char* _path = (path), *_root = (root), *_ret;     \