Allow the caller of asr functions to create and use a specific context.
Diff from eric@ and florian@, commiting on their behalf since they are absent
and we want to ride the minor shlib bump.

Remove dnssocket() and dnsconnect(), since we decided to use a SOCK_DNS
flag instead.
ok guenther tedu semarie

better placement for dnssocket/dnsconnect

Wrap <asr.h> so internal calls go direct and all the symbols are weak