unix/dgram: reduce mbuf chain traversals in send(2) and recv(2)

o Use m_pkthdr.memlen from m_uiotombuf()
o Modify unp_internalize() to keep track of allocated space and memory

as well as pointer to the last buffer.

o Modify unp_addsockcred() to keep track of allocated space and memory

as well as pointer to the last buffer.

o Record the datagram len/memlen/ctllen in the first (from) mbuf of the

chain in uipc_sosend_dgram() and reuse it in uipc_soreceive_dgram().

Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D35302