source cleanup: unify look, style and indent levels

By the use of a the new lib/checksrc.pl script that checks that our
basic source style rules are followed.

async resolvers: further cleanups

asyn-ares.c and asyn-thread.c are two separate backends that implement
the same (internal) async resolver API for libcurl to use. Backend is
specifi

async resolvers: further cleanups

asyn-ares.c and asyn-thread.c are two separate backends that implement
the same (internal) async resolver API for libcurl to use. Backend is
specified at build time.

The internal resolver API is defined in asyn.h for asynch resolvers.

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