Use a platform independent value for PKG_MAX_SIZE.

[markj]

Summary

The maximum number of rights one can send over a unix socket is
platform-dependent. Use a platform-indepdent value for the maximum in
libnv so that 32-bit applications work when running on a 64-bit kernel.
This is necessary so long as the freebsd32 compat layer incorrect
applies the 64-bit limit, which is smaller than the 32-bit limit.

Test Plan

This fixes PR 238511.