[makefile] Add -Wformat-nonliteral as an extra warning category

[makefile] Add -Wformat-nonliteral as an extra warning category

-Wformat-nonliteral is not enabled by -Wall and needs to be explicitly
 specified.

Modified the few files that use nonliteral format strings to work with
this new setting in place.

Inspired by a patch from Carl Karsten <carl@personnelware.com> and an
identical patch from Rorschach <r0rschach@lavabit.com>.