loader: add ip layer code into libstand

[tsoome]

Summary

Implement simple separate ip module and fragment re-assembly.

[bapt]

Tested on amd64 with both loader.efi and pxeboot no regression found there (testing with tftpfs)

[tsoome]

add comment to ip.c about the origins.

[tsoome]

add missing __FBSDID("$FreeBSD$");