loader is filling fixed length command_errbuf with sprintf() and is trusting
strings provided by user/config files. This update is replacing sprintf with
snprintf for cases the command_errbuf is built from dynamic content.

PR: 211958
Reported by: ecturt@gmail.com
Reviewed by: imp, allanjude
Approved by: imp (mentor), allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D7563