networkd: various fixes for the IPv6 privacy extensions support

networkd: various fixes for the IPv6 privacy extensions support

- Make sure that the IPv6PrivacyExtensions=yes results in
  prefer-temporary, not prefer-public.

- Introduce special enum value "kernel" to leave setting unset, similar
  how we have it for the IP forwarding settings.

- Bring the enum values in sync with the the strings we parse for them,
  to the level this makes sense (specifically, rename "disabled" to
  "no", and "prefer-temporary" to "yes").

- Make sure we really set the value to to "no" by default, the way it is
  already documented in the man page.

- Fix whitespace error.

- Make sure link_ipv6_privacy_extensions() actually returns the correct
  enum type, rather than implicitly casting it to "bool".

- properly size formatting buffer for ipv6 sysctl value

- Don't complain if /proc/sys isn't writable

- Document that the enum follows the kernel's own values (0 = off, 1 =
  prefer-public, 2 = prefer-temporary)

- Drop redundant negating of error code passed to log_syntax()

- Manpage fixes

This fixes a number of issues from PR #417