cli: Make cli_init() return to caller

cli: Make cli_init() return to caller

Avoid direct exit() calls as that leaves the caller-allocated nft_ctx
object in place. Making sure it is freed helps with valgrind-analyses at
least.

To signal desired exit from CLI, introduce global cli_quit boolean and
make all cli_exit() implementations also set cli_rc variable to the
appropriate return code.

The logic is to finish CLI only if cli_quit is true which asserts proper
cleanup as it is set only by the respective cli_exit() function.

Signed-off-by: Phil Sutter <phil@nwl.cc>