libpfctl: grow request buffer on ENOSPC

When we issue a request to pf and expect a serialised nvlist as a reply
we have to supply a suitable buffer to the kernel.
The required size for this buffer is difficult to predict, and may be
(slightly) different from request to request.
If it's insufficient the kernel will return ENOSPC. Teach libpfctl to
catch this and send the request again with a larger buffer.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D34908