loader: add ip layer code into libstand

Implement simple separate ip module and fragment re-assembly.

The work is based on send and receive previously implemented in udp.c,
moved to ip.c and added the ip fragment re-assembly.

This change allows to specify larger tftp or nfs payload, such as:
tftp.blksize=4096 or nfs.read_size=4096

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D10631