systemctl: rename 'listen'

src/systemctl/systemctl.c: In function ‘get_listening’:
src/systemctl/systemctl.c:535:25: warning: declaration of ‘listen’ shadows a global declaration [-Wshadow]
src/systemctl/systemctl.c: In function ‘list_sockets’:
src/systemctl/systemctl.c:690:44: warning: declaration of ‘listen’ shadows a global declaration [-Wshadow]