loader: verify the value from dhcp.interface-mtu and use snprintf to set mtu

Since the uset can set dhcp.interface-mtu, we need to try to validate the
value. So we verify if the conversion to int is successful and we will not
allow to set value greater than max IPv4 packet size.

Also use snprintf for safety.

Reviewed by: allanjude, bapt
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D8492