pfx: Add separate iterator functions for all IPv4 and IPv6 records

Add separate iterator functions to iterate through all IPv4 and IPv6
records.
If the user only want to iterate through all IPv6 records, he doesn't
have to iterate and skip through all IPv4 records before.
pfx_table: remove unnecessary asserts in iterator functions

assert(fp != NULL) was unnecessary in
pfx_table_for_each_ipv{4,6}_record(..). fp isn't dereferenced in this
function.