Replace the old OpenSSL julian date stuff with BoringSSL's

Replace the old OpenSSL julian date stuff with BoringSSL's

OpenSSL dealt with time conversion using a classical julian
day scheme. BoringSSL got rid of it and uses only a julian
style calculation for seconds since the POSIX time epoch.

This changes libressl to use the seconds calculation exculusively
instead of a mix of the julian day based conversions and the
system time conversions to and from time_t to tm.

ok tb@ jsing@