Add time callback functionality.

This commit adds `wolfSSL_SetTimeCb` and `wolfSSL_time`. The former allows the
user to override the function wolfSSL uses to get the current time,
`wolfSSL_time`. If set, `wolfSSL_time` uses that function. If not set,
`wolfSSL_time` uses the `XTIME` macro by default. This functionality is needed
for the port of chrony to wolfSSL. chrony is an NTP implementation that uses
GnuTLS by default. For TLS, chrony uses the time it computes in place of the
default system time function.