tools: bpf_dbg: various misc code cleanups

Lets clean up bpf_dbg a bit and improve its code slightly
in various areas: i) Get rid of some macros as there's no
good reason for keeping them, ii) remove one unused variable
and reduce scope of various variables found by cppcheck,
iii) Close non-default file descriptors when exiting the shell.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>