maint: `P` function prefix is for Published APIs exposed to Lua.

Naming a C function with a leading `P` indicates that it is
published to Lua.  Rename some functions that are not named
according to this scheme.
* ext/posix/posix.c (Ppoll_event_map)
(Ppoll_events_createtable, Ppoll_events_from_table)
(Ppoll_events_to_table, Ppoll_fd_list_check_table)
(Ppoll_fd_list_from_table, Ppoll_fd_list_to_table): Rename from
these...
(poll_event_map, poll_events_createtable, poll_events_from_table)
(poll_events_to_table, poll_fd_list_check_table)
(poll_fd_list_from_table, poll_fd_list_to_table): ...to these.
Adjust all callers.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>