Make temp option for ndp work

[lytboris_gmail.com]

Summary

Both ways to create an artificial NDP record are broken: a call via rtsock sets wrong timeout, a netlink code does not check opts.expire_time value at all.

Patch addresses both issues.

Test Plan

1. add 2001:DB8::/64 to an interface
2. run ndp -s 2001:DB8::c001 11:22:33:44:55:66 temp
3. check ndp -an | fgrep 2001:DB8::c001 to have < 20 minutes timeout
4. rebuild ndp with -DWITHOUT_NETLINK=1 and repeat steps 1-3

[melifaro]

Thank you for working on this! Can we add tests here?

[zarychtam_plan-b.pwste.edu.pl]

Thank you for this fix. Works fine with Netlink.