list: add list_append_ref

[cspiel1]

This allow to append an element to multiple lists

[sreimers]

Can you add some basic test code?

[cspiel1]

For now we don't need to return the le in lep. Function list_tail() could be used if it is needed. I though if we once add similar functions list_insert_xxx_ref() it would make sense to have the pointer to the new le.

[cspiel1]

I have one improvement. The flag bool ref could be added to the struct list.

[cspiel1]

We came to the conclusion that it is not worth to make list complicated in order to

• solve the issue mentioned first here: dns: add dns_rr_dup and test_dns_rr_dup #1512,
• should be solved with sip: request - fix parallel use of dnsrr #1525.
• A unit test that addresses the issue: test: dns - parallel queries #1520