For consistency with `gnrc_netif_iter()`, add the `const` qualifier to the
parameter of `netif_iter()`. This allows calling it on `const` pointers without
having to cast `netif_t *`, which (apart of dropping the `const` qualifier)
disables a lot of type safety checks offered by the compiler.
These functions are independent from GNRC and can be used by stack-agnistoc
code.
Avoid pulling in a GNRC dependency by moving those two helper functions to
`netif`.
The old function names are kept as `static inline` wrapper functions to avoid
breaking API users.
