pkg(7): replace usage of sbuf(9) with open_memstream(3)

open_memstream(3) is a standard way to obtain the same feature we do get
by using sbuf(9) (aka dynamic size buffer), switching to using it makes
pkg(7) more portable, and reduces its number of dependencies.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D30005