BUG: DTLS path has no block-wise transfer handling

[cvik]

Severity: HIGH (from external code review)

Block1/Block2 intercepts (Size1 check, Block1 reassembly, Block2 follow-up, Block2 initiation) only exist in the plain UDP handle_recv. The DTLS process_dtls_coap goes straight from parse to handler with no block-wise branches.

Fix: Mirror the block-wise handling from handle_recv into process_dtls_coap.

[cvik]

Fixed in PR #89 — shared block helpers wired into process_dtls_coap.