dtls: change to state closing when sending a fatal alert
dtls: change to state closing when sending a fatal alert

When we send a fatal alert we can not recover and a new connection
should be established.

Signed-off-by: Hauke Mehrtens <435ddd46dc66c007a1fa20144c823d37e0d23436@hauke-m.de>

thecodemaiden/tinydtls-fork,nmacs/tinydtls,thecodemaiden/tinydtls-fork,yunus/TinyDTLS-WebID,OlegHahm/tinydtls,nmacs/tinydtls,fvdnabee/tinydtls,jvermillard/tinydtls,fvdnabee/tinydtls,yunus/TinyDTLS-WebID,jvermillard/tinydtls,OlegHahm/tinydtls,nmacs/tinydtls,fvdnabee/tinydtls,spark/tinydtls,thecodemaiden/tinydtls-fork,jvermillard/tinydtls,nmacs/tinydtls,OlegHahm/tinydtls,spark/tinydtls,spark/tinydtls,yunus/TinyDTLS-WebID