Refactor: get rid of booth_transport as a global var&finish booth_config

Finish ~ finally mark static.

As for booth_transport, it is still more covenient to split definition
and actual use across two files (transport.c and main.c, respectively),
so we at least make the semi-hidden nature (no presence in any of the
actual header files) apparent with the double-underscore after initial
namespace prefix convenience.

Also fix a latent "no extern storage class specifier specified" in case
of compiling with -fno-common/default since GCC 10 (for missing
"extern" qualifier).

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>