routine file_check_access (
    file: file_t;
    RPT
    out allowed: int);
Find out what kind of access this file permits the current user (regardless of
the current open modes for this port).  allowed is a bitwise or of
O_READ, O_WRITE, and O_EXEC.  This is not necessarily the same as what an
open or exec would allow; O_EXEC is set for root even if no executable
bits are on (in which case file exec should fail) and O_WRITE
is set a directory can be modified, even though it can't be written directly.