Use doas instead of sudo if it's installed (fixed)

[StratusFearMe21]

Fixes the issues in #323. Same concept as the original

[morgant]

Thanks for your improvements, @StratusFearMe21! I'm not an official maintainer here or anything, but I've been maintaining a personal OpenBSD port and have proposed some changes to increase portability to BSDs (see Issue #344 & PR #348).

So, the following suggestions come from using doas under OpenBSD and my efforts to improve portability, but also to minimize changes for @graysky2 to review and later maintain:

• I'd lean toward using only SUDO and just setting it to either sudo or doas and using it as the binary to execute
• It seems like a reasonable requirement that $SUDO should be in PATH and could be confirmed with which in dep_check()
• doas does support the -n option, so probably best to retain that so that profile-sync-daemon doesn't try to prompt for authentication interactively
• The change to use runuser instead of sudo is not portable, so I'd suggest continuing to use $SUDO -u in its stead

Of course, that still leaves the sudo -kn situation. I submit the following for consideration:

• While doas does not support sudo's -k option, it does have a -L option which appears to be equivalent to sudo -k without a command (doas -L exits without running a command)
• While it would change the functionality by invalidating cached credentials for the session, instead of just ignoring cached credentials while executing the command, by separately calling sudo -k before sudo -n [...] we could more easily & cleanly conditionally execute either $SUDO -k or $SUDO -L depending on the value of $SUDO
• That would also allow $SUDO -n [...] to be used without adding further conditionals

I'm curious regarding your thoughts. I'm happy to put together an alternate PR, but wouldn't be able to until Sunday at the earliest.

[dorsiflexion]

@graysky2 Any chance this gets merged? This seems like not a big change and thus doable. I could help make this PR even shorter and more in line with the rest of the code, if you wish.

[graysky2]

I never used doas before. Need to test. Do you mind squash/rebase into a single commit?