net: shell: udp: allow sending packet when bound

"udp bind" and "udp send" commands use the same net context
variable and they fail early if the context is already used.

This prevents from using "udp send" after "udp bind", which
makes the commands hard to use for testing bidirectional
communication. Make "udp send" reuse the already bound
context if possible, and resort to allocating temporary one
otherwise.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>