Replace atoi with explicit checks; rm unused atof

Replace atoi with explicit checks; rm unused atof

Replace atoi with explicit checks, making the intention a bit clearer
at each call site, and avoiding trouble we've had in the past with the
default to 0 for any unrecognized input.

This does make us stricter about the values (and the environment),
i.e. BUP_TTY_WIDTH=random-noise will now cause a crash.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>