target/iscsi: Fold _decide_list_to_build into _build_pdu_and_seq_lists

Rename iscsit_build_pdu_and_seq_list to iscsit_do_build_pdu_and_seq_lists

Rename iscsit_do_build_list to iscsit_build_pdu_and_seq_lists

Move code from iscsit_decide_list_to_build into _seq_pdu_list.c, seems
a better fit.

Also update some comments in pdu/seq code for correctness and whitespace.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>