reference holders that their reference is about to become invalid.

"boot" command to attempt booting from within the command shell, fall back
to shell if boot fails for any reason.

the option config UI.

Introduce new {show,set,clear}_setting() that take a struct setting *
rather than a const char *.

set_setting() handles calling clear_setting() when appropriate, so that
individual setting types don't have to check for empty strings.

memory" and "no space left for DHCP options"

command is linked in.

non-ANSI-capable consoles.

in our commands.

completely forgot I'd written ages ago.

device.

Separated the mechanisms of non-volatile storage access and non-volatile
stored options.

for realloc(0) being a valid way to free memory.

with octal yet, but we can probably live without that.

defined in vsprintf.h.  (This may change, since vsprintf.h is a
non-standard name, but for now it's the one to use.)

There should be no need to include vsprintf.h just for DBG() statements,
since include/compiler.h forces it in for a debug build anyway.