Introduce a new function parse_hn_line() that replaces the existing

Introduce a new function parse_hn_line() that replaces the existing
hostname.if(5) parsing code in ifstart().

- change v6_config()
  - replace rtsol with autoconf in "IPv6 addres for <if>" question
  - write 'inet6 autoconf" to hostname.if file instead of rtsol
- add parse_hn_line() function with installer specific changes
  - skip shell command and bridge config lines
  - skip inet6 config (incl. dest lines) if there's no inet6 support
  - skip dhcp config if there's no dhclient
- change ifstart() to use parse_hn_line()

Discussed with and positive feedback from many
'commit' deraadt@
conditional OK sthen@ (untested)