correct server random format

The ServerHello message uses the normal Random accordingly to RFC5246,
7.4.1.3. Server Hello. This Random attribute consist of 4 bytes Unix
time stamp followed by 28 bytes of random data, see RFC5246, 7.4.1.2.
Client Hello.
do not generate random which is replaced by timestamp

The first 4 bytes of this are overwrite with the timestamp.
install target in Makefile
This call was missing, but it worked somehow with the current code.
make DTLSv12 the default
automatically connect in dtls_write()
suppress debug output when -DNDEBUG is defined (see configure option --without-debug)
started cleanup of dtls.[hc]; new function dtls_connect_peer()
moved time functions to dtls_time.[hc] and renamed them
renamed set/get_log_level to dtls_set_log_level()/dtls_get_log_level()
removed debug output
fixed things that are needed for DTLSv1.2 (still seems not to work, though)
code cleanup: remove a lot of noisy debug output
bugfix: release storage for read_cipher/write_cipher (thanks to Mališa Vučinić for pointing this out)