libc/stdio: Allow fopen/freopen modes in any order (except initial r/w/a).

Austin Group issue #411 requires 'e' to be accepted before and after 'x',
and encourages accepting the characters in any order, except the initial
'r', 'w' or 'a'.

Given that glibc accepts the characters after r/w/a in any order and that
diagnosing this problem may be hard, change our libc to behave that way as
well.