loader command interpreter should reset command_errmsg

The command interpreter does leave command_errmsg as is after printing its
content, assuming the next command will reset it in bf_command(). However,
in case the forth native word is defined as builtin, the bf_command is not
used and forth words will also end up the command_errmsg content printed.

Since command_errmsg is pointer to actual error message, which can be static
read only string, we can not just set *command_errmsg = '\0', instead we need
to reset the pointer itself.

Illumos issue: https://www.illumos.org/issues/7405

Reported by: Igor Kozhukhov.
Reviewed by: allanjude
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D8032