This gnulib feature abstracts away threads, locks and TLS, and also
allowed libguestfs to be linked with or without pthread. However
since pthread these days is part of glibc and so every program is
using pthread, and we want to get rid of gnulib as a dependency, just
use pthread directly.
Acquire the per-handle lock on entering each public API function.
The lock is released by a cleanup handler, so we only need to use the
ACQUIRE_LOCK_FOR_CURRENT_SCOPE macro at the top of each function.
Note this means we require __attribute__((cleanup)). On platforms
where this is not supported, the code will probably hang whenever a
libguestfs function is called.
The only definitive list of public APIs is found indirectly in the
generator (in generator/c.ml : globals).
