Rename (again) the tuntap-sys-* files to tuntap_start-*.c

Now that almost all of them (except Windows) only hold tuntap_sys_start
it is better to give them a proper name.
Add a tiny style guide.
Add a sentence, no real differences.
Add documentation about compatibility
Properly inform in case of persistence on any

It's an TUNTAP_MODE_PERSIST is set with TUNTAP_ID_ANY, this diff just
explain it trough tuntap_log().
Add a doc/examples folder with a first demo
OpenBSD seems to support if description up to 64 c

Test can be replayed like this:
    ifconfig tun0 description "`jot -s '' -r -c 63 a {`"
Add a comment about tuntaposx.
Move ETHER_ADDR_LEN to tuntap.h and add SunOS compat
Make basic files compile on Solaris 10
Add Solaris handling in CMakeLists.txt