Be more compatible with SysV sh

Although $((arithmetic expansion)) is POSIX-specified, some systems have
non-POSIX System V shell, which can't handle it. This patch replaces
$((something)) with $(expr something) to fix compatibility. This
actually slows things down considerably, since a subshell must be
launched for each increment operation, but the tests aren't that
critical. Bug report: http://seclists.org/nmap-dev/2013/q4/198