Use a platform-independent constant for PKG_MAX_SIZE.

This constant determines the number of rights libnv will attempt to
transmit in a given control message. In practice, the upper limit
defined by the kernel is machine-dependent and is smaller on 64-bit
kernels than on 32-bit kernels. To ensure that a 32-bit libnv works
as expected when run on a 64-bit kernel, use a limit that will work
on both 32-bit and 64-bit kernels.

PR: 238511
Discussed with: oshogbo
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20942