Add an oath_mode(3) function which translates from mode names to numbers.
Consistently use UINT_MAX, not -1, to indicate an invalid response.
Change the meaning of the window parameter to always indicate the number
of codes to check *in addition* to the current code. Note that for TOTP,
the window goes in both directions; a window of 1 means to check the
current code plus the previous and next.
