Wrap <resolv.h> so that internal calls go direct

ok millert@

Make the asr API public. Install asr.h to /usr/include.h and manpages.
Include tweaks suggested by mpi@

ok deraadt@

Cleanup and simplify the API to be exposed. Use better names for
structures, functions and defines.

discussed with and ok deraadt@ guenther@

pull in missing includes (math.h, unistd.h, stdlib.h) needed for
prototypes

Make some symbols static and prefix all visible symbols with asr_
to prevent collisions with third-party programs.

suggested by sthen@, ok theo@

Add minimal support for _res setup and update.

Change res_init() to initialize the _res structure on first call, and
udpate the global async context if changes were made by the user. All
resolver fu

Add minimal support for _res setup and update.

Change res_init() to initialize the _res structure on first call, and
udpate the global async context if changes were made by the user. All
resolver functions now call res_init() internally.

fixes issue spotted by otto@

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Do not take external buffers for storing DNS responses in the internal
async API. That is actually useless and it makes the code more
complicated. The required buffers are always allocated when nee

Do not take external buffers for storing DNS responses in the internal
async API. That is actually useless and it makes the code more
complicated. The required buffers are always allocated when needed.

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Do not fail if the user buffer is too short to hold the packet: fill it up
to the given size and return the packet length.

issue spotted by weerd@

split asr_resolver.c into different files to overlay the libc/net
resolver implementation.