Add an asynchronous resolver library.

src/Makefile.am: Add sar.* to the build.
src/main.c: Call sar_init().
src/sar.*: The srvx asynchronous resolver.
src/mod-blacklist.c: Use it to add DNSBL support.
srvx.conf.eaxmple: Document the DNSBL support and sar options.