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().
Add a -n option to netstart to only print the interface configuration
commands instead of executing them.
Add a HN_DIR variable, that points to the directory of the hostname.if
files (default /etc) that allows for future regression tests.

- add new parse_hn_line() function
- change ifstart()
  - rename $if to $_if
  - don't ifconfig or ifconfig create if -n option is used
  - replace hostname.if(5) parsing code with new parse_hn_line()
  - just print configuration commands if -n option is used
- autoconf now happens in ifstart(), remove ifv6autoconf()
- introduce HN_DIR variable for the hostname.if file location
- add handling of the -n option to only print config commands
- ensure -n is only used if interfaces are specified as parameters

Discussed with and positive feedback from many
'commit' deraadt@
OK sthen@