Fix some issues with printing (make-pathname :directory '(:relative)),
which used to print as #p"", and with reading #p".", #p"./".
code/filesys.lisp:
o When parsing a unix namestring, delete any "." elements of the
directory list.
o If we've parsed a namestring such that the file name is ".", replace
that with :name NIL and adjust the :directory component
appropriately, because on Unix, "." can't be the name of a file.
o Make :directory '(:relative) be printed as "./"
code/pathname.lisp:
o If the :directory argument to MAKE-PATHNAME contains strings with
#\/, print a warning that this is not a valid element.
o Remove all "."'s from a :relative directory component.