Improve syscall wrappers with FIELD macros

[paulfloyd]

Sadly because the old port never tracked upstream closely a lot of stuff never got done on FreeBSD.

One of those is the use of PRE_FIELD_READ and POST_FIELD_WRITE in syscall wrappers. These allow better error messages (not only the syscall argument but also the field within the structure that the syscall argument points to).