io.files: make wait-for-fd generic and dispatch depending on backend

do-ssl-accept need to call wait-for-fd so it needs to be generic. On
windows it's a noop but on unix it will wait for the fd.

basis/io/backend/unix/unix.factor

@@ -69,7 +69,7 @@ ERROR: io-timeout ;
 
 M: io-timeout summary drop "I/O operation timed out" ;
 
-: wait-for-fd ( handle event -- )
+M: unix wait-for-fd ( handle event -- )
     dup +retry+ eq? [ 2drop ] [
         [ [ self ] dip handle-fd mx get-global ] dip {
             { +input+ [ add-input-callback ] }

basis/io/files/windows/windows.factor

@@ -206,6 +206,9 @@ M: windows (wait-to-write) ( port -- )
 M: windows (wait-to-read) ( port -- )
     [ dup handle>> refill ] with-destructors drop ;
 
+M: windows wait-for-fd ( handle event -- )
+    2drop ;
+
 : console-app? ( -- ? ) GetConsoleWindow >boolean ;
 
 M: windows init-stdio

basis/io/sockets/secure/openssl/openssl.factor

@@ -202,13 +202,10 @@ SYMBOL: default-secure-context
         { SSL_ERROR_SSL [ (ssl-error) ] }
     } case ;
 
-! Does this work on windows?
 : do-ssl-accept ( ssl-handle -- )
     dup dup handle>> SSL_accept check-accept-response dup
     [
-        [ dup file>> ] dip
-        2drop ! wait-for-fd, todo: figure out wait-for-fd for windows
-        do-ssl-accept
+        [ dup file>> ] dip wait-for-fd do-ssl-accept
     ] [ 2drop ] if ;
 
 : maybe-handshake ( ssl-handle -- )

core/io/files/files.factor

@@ -14,6 +14,8 @@ SYMBOL: +output+
 GENERIC: drain ( port handle -- event/f )
 GENERIC: refill ( port handle -- event/f )
 
+HOOK: wait-for-fd ( handle event -- )
+
 MIXIN: file-reader
 MIXIN: file-writer