Check for major upgrades to be run with sysupgrade

[morgant]

It would be convenient to check and notify if a newer major OpenBSD version is available to be installed with sysupgrade.

I don't believe sysupgrade has an option to just check if an update is available. It does, however, have an -n option to download the files, but not perform the upgrade. Naturally, it would be important to check for release vs snap upgrades.

[morgant]

Now that I've implementing checking for when running a kernel that is in the small post-beta/pre-release window (see Issue #8), I think I could use a similar methodology to check for newer major upgrades being available: check for the next kernel version path on the mirror and ensure that the SHA256.sig and architecture-specific upgrade files are available. Fortunately, OpenBSD always rolls over release versions from X.9 to (X+1).0, so the next release is easy to predict.

[morgant]

I've got this functional (added a new check_for_sysupgrade() function and some additional logic when using the swupdate -i option), but it needs a bit more testing.

[morgant]

A few more tweaks and this is now complete.

[morgant]

I have updated the new check_for_sysupgrade() function to:

1. Skip checking for upgrades:
   • If running snapshots (probably preferable to just run sysupgrade manually in such circumstances anyway)
   • If sysupgrade is unavailable
2. Urge user to review appropriate release notes, errata, and the upgrade guide before running sysupgrade if a new release is available

Now this should be done.