dispatcher: refactor constructing environment variables from strv

dispatcher: refactor constructing environment variables from strv

This also avoids warnings with -O2 -Wstrict-overflow (gcc-4.8.3):

    make[4]: Entering directory `./NetworkManager/callouts'
      CC       libtest_dispatcher_envp_la-nm-dispatcher-utils.lo
    nm-dispatcher-utils.c: In function 'construct_ip6_items':
    nm-dispatcher-utils.c:283:8: error: assuming signed overflow does not occur when simplifying conditional to constant [-Werror=strict-overflow]
         if (i != 0)
            ^
    nm-dispatcher-utils.c: In function 'construct_ip4_items':
    nm-dispatcher-utils.c:144:8: error: assuming signed overflow does not occur when simplifying conditional to constant [-Werror=strict-overflow]
         if (i != 0)
            ^
    nm-dispatcher-utils.c:168:8: error: assuming signed overflow does not occur when simplifying conditional to constant [-Werror=strict-overflow]
         if (i != 0)
            ^

https://bugzilla.gnome.org/show_bug.cgi?id=741168