odhcp6c: enable Non-Blocking DHCPv6 Socket

odhcp6c: enable Non-Blocking DHCPv6 Socket

Problem:
    The DHCPv6 socket is currently set to blocking mode, which causes
    the program to pause execution whenever it attempts to read from the
    socket. This can lead to delays and hinder the overall
    responsiveness of the application.

Solution:
    Add the set_nonblocking() function, using fcntl() to set the
    O_NONBLOCK flag, allowing the socket to operate in non-blocking
    mode.

Signed-off-by: Nicolas BESNARD <[email protected]>
Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcp6c/pull/106
Signed-off-by: Álvaro Fernández Rojas <[email protected]>