bsdinstall partedit: Use asprintf to build wrapper command for newfs

Don't abuse the message[] static buffer used elsewhere for error
messages to generate the command that actually newfs's each
filesystem. Use asprintf to a more aptly-named 'char *command'
variable to construct the string instead. This avoids potential bugs
from truncation of the command string.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D42239