port position-tracking clean-ups

Add `file-position*', which can return #f instead of raising
an exception when a port's position is unknown. Change
`make-input-port' and `make-output-port' to accept more
kinds of values as the initial position.

These changes make it possible to synchronize a port's
position with a `port-commit-peeked' action. It's ugly,
which I think reflect something broken about position
tracking in the port protocol (which seems difficult to fix
without breaking compaibility).

original commit: b61f3f751c5dfe72869fd449ba3ff42f520d0927

collects/typed-racket/base-env/base-env.rkt

@@ -2155,6 +2155,7 @@
                                [(-Port (one-of/c 'none 'line 'block)) -Void])]
 [file-position (cl-> [(-Port) -Nat]
                      [(-Port -Integer) -Void])]
+[file-position* (-> -Port (Un -Nat (-val #f)))]
 
 ;Section 12.1.4
 [port-count-lines! (-> (Un -Input-Port -Output-Port) -Void)]