The integer part is the number of days to be added to the epoch
(1899-12-30), and the fractional part (disregarding sign) represents
the fraction of that day that has elapsed.

For 0.0 < x < 1.0, DATE values x and -x represent the same date/time:
30-Dec-1899 at some time determined by x.  When converting from DATE,
either value is accepted.  When converting to DATE, the positive value
is produced.

Spotted by André Hentschel.

crypt32: Set correct error when encountering unsupported critical extensions in the base and SSL policy.

crypt32/tests: Test the base and SSL policies against a certificate with an invalid critical extension.