Support ffi.dlopen(<void* cdata>).  See updated documentation.

Now the with non pkg-config backup would be
```
module_name = "_czmq"
source = "#include <czmq.h>"
try:
    print(f"### pkg-config path")
    ffibuilder.set_source(
        module_name,
        source,
        pkgconfig=["libczmq"]
    )
except Exception as e:
    print(f"Exception e: {e}")
    ffibuilder.set_source(
        module_name,
        source,
        libraries=["czmq"]
    )
```

Fix encoding errors

Given testing Python program
```
from cffi import FFI
ffibuilder = FFI()

ffibuilder.cdef(
    "char* zsys_hostname();"
)

ffibuilder.set_source(
    "_czmq",
    "#include <czmq.h>",
    pkgconfig=["libczmq"]
)


if __name__ == "__main__":
    ffibuilder.compile(verbose=True)
```

We can run ffibuilder from source dir of czmq

```
PKG_CONFIG_PATH=`pwd`/src python3 t.py
generating ./_czmq.c
...
gcc -pthread -shared -flto -fuse-linker-plugin -ffat-lto-objects -flto-partition=none ./_czmq.o -L/usr/local/lib64 -L/usr/lib64 -lczmq -lzmq -lpython3.6m -o ./_czmq.cpython-36m-x86_64-linux-gnu.so
```

```
python3 t.py
generating ./_czmq.c
...
gcc -pthread -shared -flto -fuse-linker-plugin -ffat-lto-objects -flto-partition=none ./_czmq.o -L/usr/lib64 -lczmq -lpython3.6m -o ./_czmq.cpython-36m-x86_64-linux-gnu.so
```

Note that in the first case `/usr/local` has been added to the compiler path as
provided by local pkg-config file.

Making `pkgconfig.call` function accessible, tests can monkey patch it and
provide mock. This improves testing, however raised a need to give
functions better names than `pkgconfig.pkgconfig_kwargs` or `pkgconfig.pc`.

Also contains some improvements to the documentation about other recent
additions