ecx_statecheck() when called with the first argument set to 0 automatically updates the state of all the slaves in the slavelist structure when possible,
`ifname` as the argument of `ec_init` and related functions is not
changed, and therefore can be safely retyped to `const char *`. This
allows using string literals. For example:
ec_init(argc > 1 ? argv[1] : "eth0");
Signed-off-by: Shahbaz Youssefi <syoussefi@kinova.ca>