In some harmless places, checks for the return value of ECDSA and RSA
key generation and verification was omitted. Add them to keep the
compiler happy and to warn end users in case something is wrong.
When using socket functions, "sockerrno" is supposed to be used to
retrieve the error code as opposed to "errno", so that it is translated
to the correct call on Windows (WSAGetLastError() - Windows does not
update errno on socket errors). Unfortunately, the use of sockerrno is
inconsistent throughout the tinc codebase, as errno is often used
incorrectly on socket-related calls.
This commit fixes these oversights, which improves socket error
handling on Windows.
