Rewrite, fix and improve str2net().

This is a complete rewrite of the str2net() function. Besides
refactoring duplicate code, this new code brings the following fixes
and improvements:

 - Fixes handling of leading/trailing double colon in IPv6 addresses.
   For example, with the previous code the address
   2001:0db8:85a3:0000:0000:8a2e:0370:: is interpreted as a MAC address,
   and ::0db8:85a3:0000:0000:8a2e:0370:7334 is rejected.

 - Catches more invalid cases, such as garbage at the end of the string.

 - Adds support for dotted quad notation in IPv6 (e.g. ::1.2.3.4).

See RFC 4291, section 2.2 for details on the textual format of IPv6
addresses.
When printing MAC addresses, always use trailing zeroes.

tinc currently prints MAC addresses without trailing zeroes, for example:

    1:2:3:4:5:6

This looks weird and is inconsistent with how MAC addresses are
displayed everywhere else. This commit adds trailing zeroes, so the
above address will be printed as the following:

    01:02:03:04:05:06
Don't print subnet prefix lengths and weights for one-host subnets.

This commit suppresses subnet prefix length output (/xx) for subnets
that only contain one address (/32 for IPv4, /128 for IPv6). It also
suppresses weight information if the subnet is using the default
weight. This improves readability of net2str() output in the majority
of cases.
Canonicalize IPv6 addresses as per RFC 5952 before printing them.

Currently we don't do any shortening on IPv6 addresses (aside from
removing trailing zeroes) before printing them. This commit makes
textual addresses smaller by shortening them according to the rules
described in RFC 5952. This is also the canonical textual representation
for IPv6 addresses, thus making them easier to compare.
Fix tinc event loop reentrancy from timeout handlers.

Commit 611217c96ec684799882cf330f40a0936131b6b5 introduced a regression
because it accidentally reordered the timeout handler calls and the
fdset setup code. This means that any io_add(), io_del() or io_set()
calls in timeout handlers would be ignored in the current event loop
iteration, resulting in erratic behavior.

The most visible symptom is when a metaconnection timeout occurs and the
connection is closed; the timeout handler closes the socket but it still
ends up in the select() call, typically resulting in the following
crash:

    Error while waiting for input: Bad file descriptor