truss: Add preadv(2) and pwritev(2).

[tmunro]

Summary

We already knew how to decode readv(2)/writev(2). Add the versions with an offset.

Test Plan

Check output from simple programs. Example:

78813: preadv(168,[{"\0\0\0\0\M-8gR\^A\0\0\0\0\^X\0\0"...,8192}],1,0x0) = 8192 (0x2000)
75958: pwritev(13,[{"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,8192},{"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,8192}],2,0x40000) = 16384 (0x4000)