SSL: backed out changeset e7cb5deb951d, reimplemented properly.

Changeset e7cb5deb951d breaks build on CentOS 5 with "dereferencing
type-punned pointer will break strict-aliasing rules" warning.  It is
backed out.

Instead, to keep builds with BoringSSL happy, type of the "value"
variable changed to "char *", and an explicit cast added before calling
ngx_parse_http_time().