url_parse(): better take better care for path parts (Stephen Isard)..

url_parse(): better take better care for path parts (Stephen Isard)..

Stephen reported

  I have an imap account to which I can connect with mailx but not s-nail.
  In the account definition in my .mailrc, I have
  account XX {
          set folder=imaps://username@servername/Mail
..
  Running s-nail with -vv I see that I am logging in successfully with
  gssapi, but then
  T2 OK Logged in
  >>> T3 SELECT "Mail/"
  T3 NO [CANNOT] Invalid mailbox name
..
  Anyway, mailx is doing the right thing for that imap server, but s-nail
  is not.

url_parse() yet doesn't normalize the input (i.e., splits it up
into its components and joins them correctly as requested).
This results in problems when there are adjacent slashes as well
as for single-slash URLs as above.