1.0.44.21: expand ~ in pathnames

1.0.44.21: expand ~ in pathnames

  ~/... => (:ABSOLUTE :HOME ...)

  ~user/... => (:ABSOLUTE (:HOME "user") ...)

  Translation back to NAMESTRING reinstates the tilde, so we retain
  read/write consistency.

  NATIVE-NAMESTRING is responsible for getting the actual full path
  to specified home directory.

  This late resolution is necessary to have (open "~/foo") and
  (open #p"~/foo") open the same file in compiled code -- regardless
  of who compiled the file.

  Tilde is treated specially only at the start of the first directory
  component: it doesn't need to be escaped anywhere else. After trying
  out the various options (escape everywhere, escape in directory
  components, escape at the start of directory components, escape at
  the start of all components) this seemed both least intrusive and
  least ambiguous when documented -- not to mention most backwards
  compatible.

  Currently escaping the tilde does not work on Windows, but this is due to
  current general inability to escape the first directory component on
  Windows, since \\ is used also as a directory separator for non-native
  pathnames as well. See lp#673625. Test-case added for this.

  (:HOME "user") also doesn't work on Windows, which is documented
  in the manual.