base-files: ipcalc.sh: use shebang to invoke awk

There's hardly an shell logic in ipcalc.sh and a $* that would garble
parameter positions.
Move the awk invokation to the shebang.

A rename from "ipcalc.sh" to "ipcalc" is desirable but could prove tricky
with packages in other repositories depending on the filename.

Signed-off-by: Leon M. George <leon@georgemail.eu>