feat(server): instance "name", store pipes in stdpath(state)

Problem:
- Unix sockets are created in random /tmp dirs.
  - /tmp is messy, unclear when OSes actually clear it.
  - The generated paths are very ugly. This adds friction to reasoning
    about which paths belong to which Nvim instances.
- No way to provide a human-friendly way to identify Nvim instances in
  logs or server addresses.

Solution:
- Store unix sockets in stdpath('state')
- Allow --listen "name" and serverstart("name") to given a name (which
  is appended to a generated path).

TODO:
- is stdpath(state) the right place?

src/nvim/path.c

@@ -88,7 +88,12 @@ FileComparison path_full_compare(char_u *const s1, char_u *const s2, const bool
   return kDifferentFiles;
 }
 
-/// Gets the tail (i.e., the filename segment) of a path `fname`.
+/// Gets the tail (filename segment) of path `fname`.
+///
+/// Examples:
+/// - "dir/file.txt" => "file.txt"
+/// - "file.txt" => "file.txt"
+/// - "dir/" => ""
 ///
 /// @return pointer just past the last path separator (empty string, if fname
 ///         ends in a slash), or empty string if fname is NULL.