Introduce generic linked list implementation

[Cropi]

I’m wondering if it might be beneficial to have a generic linked list implementation, as proposed in this PR. While I understand that multiple implementations exist to serve specific needs, introducing a shared, flexible version could simplify the codebase. A quick grep shows the following similar files that could be handled in this fashion:

audisp/audispd-llist.c
audisp/audispd-llist.h
audisp/plugins/ids/nvpair.c
audisp/plugins/ids/nvpair.h
auparse/ellist.c
auparse/ellist.h
auparse/normalize-llist.c
auparse/normalize-llist.h
auparse/nvlist.c
auparse/nvlist.h
src/auditctl-llist.c
src/auditctl-llist.h
src/ausearch-avc.c
src/ausearch-avc.h
src/ausearch-int.c
src/ausearch-int.h
src/ausearch-llist.c
src/ausearch-llist.h
src/ausearch-nvpair.c
src/ausearch-nvpair.h
src/ausearch-string.c
src/ausearch-string.h
tools/aulast/aulast-llist.c
tools/aulast/aulast-llist.h
tools/aulastlog/aulastlog-llist.c
tools/aulastlog/aulastlog-llist.h