update p_get/set_metadata to use $INFO['meta']

This patch updates p_get_metadata() to utilise the in memory copy
copy of the current page's metadata ($INFO['meta']) when appropriate.

The patch also updates p_set_metadata() to synchronise any changes
to the current page's metadata with $INFO['meta'].

$INFO hash is updated with two new elements:
  'id'

darcs-hash:20061004210030-9b6ab-7eab6f933a775fe350a1fb14d1118ea77d2db919.gz