iptables-apply: Eliminate shellcheck warnings

iptables-apply: Eliminate shellcheck warnings

Actual warnings were only about use of '-a' in bracket expressions
(replace by '&&' pipeline) and the immediate evaluation of the variable
in trap command.

The remaining changes silence info-level messages: missing quoting
around variables, pointless '$' in arithmetic expressions, backticks
instead of $(...), missing '-r' parameter when calling read and an
awkward negated '-z' check.

Signed-off-by: Phil Sutter <phil@nwl.cc>