iprange: rework ip range parsing functions

The previous parsing functions had a few problems:

  * They did accept negative numbers in addresses (e.g. 192.0.2.-5).

  * They relied on C undefined behaviour for detecting /0 prefix
    length: "mask = htonl(~((1 << (32 - m)) - 1)" was wrong for m = 0,
    because that resulted in a left shift of 32 bits, on a 32 bit wide
    value (the right operand of a bitwise shift operator must be
    strictly smaller than the width of the promoted left operand).

  * They misinterpreted /32 prefixes as disable requests. In fact, due
    to the undefined behaviour described above, /0 and /32 prefix
    lengths were represented in the same way by parse1(), that is, with
    an iprange_t structure where ->begin == ->end. Therefore
    load_ranges() had no way to distinguish between them and did
    disable the module in both cases.

This patch fixes these issues and brings the following improvements:

  * It uses getaddrinfo() to parse IP addresses, so it accept (almost)
    all IPv4 representations and is more easily extensible to IPv6 in
    the future.

  * It warns when the IP address used in CIDR notation is not the first
    address in the range (e.g. the first address of 192.0.2.1/24 is
    192.0.2.0, not 192.0.2.1).

  * It doesn't _exit() on parsing failures, thus making the functions
    usable in an EV_CONFIG_RELOAD handler.

While there, the unfinished tunnel_ranges code, which was already
commented, has been removed.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>

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