lib-dns: Allow setting the ioloop that the dns_lookup/dns_client is started on.

lib-dns: Added dns_client_switch_ioloop().

lib-dns: Added alternative API for doing longer connections to dns-client process.

dns, lib-dns: Added support for async DNS PTR lookups.

lib-dns: Added DNS_CLIENT_SOCKET_NAME macro for the default socket name.

lib-dns: Added dns_lookup_switch_ioloop()

Removed CONTEXT_TYPE_SAFETY macro and reimplemented its functionality better. gcc/clang now gives a compiler error in many places if callback isn't exactly what was expected. It's also now much easier to add more of these checks.

Marked functions parameters that are allowed to be NULL. Some APIs were also changed. The non-obvious APIs where NULL parameter was changed to "" are master_service_init() and auth_master_user_list_init(). These checks can currently be enabled only on a patched clang: http://llvm.org/bugs/show_bug.cgi?id=6786

lib-dns: dns_lookup() returns now the lookup struct, and it can be aborted. Changed all dns_lookup() users also to abort the lookup when needed (previously it probably would have just accessed freed memory and crash).

Added dns-client service and library for doing async dns lookups. --HG-- branch : HEAD