dtls: use a negative number to indicate an error
dtls: use a negative number to indicate an error

In preparation to convert a error code into a TLS alert message convert
the code to indicate a success with 0 or a positive number and an error
with a negative number.

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

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