bsdinstall partedit: Avoid potential buffer overflow in newfs_command

Allocate the buffer holding the newfs command string dynamically
(building the string via open_memstream) rather than storing the
command into a caller-supplied buffer of unknown length.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D42237

(cherry picked from commit ae2fc74fe76ca8b89c5ef0081ef3f4008f83de41)