Add macros to make code compile in kernel

Make it possible to compile libnv in the kernel. Mostly this
involves wrapping functions that have a different signature in
the kernel and in userland (e.g. malloc()) in a macro that will
conditionally expand to the right API depending on whether the
code is being compiled for the kernel or not.

I have also #ifdef'ed out all of file descriptor-handling code,
as well as the unsafe varargs functions.

Differential Revision: https://reviews.freebsd.org/D1882
Reviewed by: jfv
MFC after: 1 month
Sponsored by: Sandvine Inc