package: avoid the use of eval to parse ipcalc.sh output

package: avoid the use of eval to parse ipcalc.sh output

Add a function 'ipcalc' to /lib/functions.sh that sets variables more
safely using export.
With this new function, dnsmasq also handles the return value of ipcalc
correctly.

Fixes: e4bd3de1be8e ("dnsmasq: refuse to add empty DHCP range")
Co-Authored-By: Philip Prindeville <philipp@redfish-solutions.com>
Signed-off-by: Leon M. Busch-George <leon@georgemail.eu>