Changes to support endpoints.
. rename message field name macros from *PROC* to *ENDPT*,
both to reflect the new meaning and to use it to hunt down where
they are used
. _PM_SEG_FLAG in fd replaces funny segment loading construction by
PM in FS
. _MAX_MAGIC_PROC is the highest used proc number, including magic
constants such as (currently) ANY, NONE and SELF, used by the
endpoint macros to make sure they are encoded properly and no valid
endpoint number ever encodes to it.
. rename proc_nr in address copy struct to proc_nr_e (endpoint)