[libc] Allow out-of-range months in mktime()

POSIX specifies that the values of members of the broken-down time
structure are "not restricted to the ranges", and defines the way in
which out-of-range values are to be handled.

For most fields, the arithmetic is already purely linear and so
out-of-range values are handled automatically.  Out-of-range months
are an exception: these are used as array indices and so must be
normalised before use.

Restructure mktime() to make it more immediately visible when values
are being read from and written back to the broken-down time
structure, add the required normalisation for the month number, and
add test cases to cover out-of-range months.

Signed-off-by: Michael Brown <mcb30@ipxe.org>