(PUP-2578) Unbreak services from packages which set local flags.

So don't add _flags="" for pkg_scripts if no :flags were set, as it
may override the default flags from inside the script.
(PUP-2578) Keep order for pkg_scripts.

Do not unconditionally alphabetically sort the list of pkg_scripts
as order matters. Explicit ordering will need to be forced through
resources dependencies.
(PUP-2578) Tweak #in_base? by not relying on pkg_info, but on /etc/rc.conf which is faster
(PUP-2578) Adjust spec tests to recent changes:

- Ensure strict ordering of the result of #pkg_scripts_append
- Add test for #in_base?
- Ensure package scripts don't end up with empty flags
Merge pull request #2557 from jasperla/service

(PUP-2578) Adjustments for new OpenBSD service provider