Fix misc compiler warnings

[jschmidt-icinga]

Together with #9411, #9728, #9729 and #9730 this fixes most of the remaining compiler warnings, except the following:

• A few -Wdangling-reference in scheduleddowntime-apply.cpp, which I'm pretty sure are false positives.
• The OpenSSL 3.0 -Wdeprecated-declarations warnings.
• The '-Wdeprecated' warnings about %pure-parser being deprecated in the .yy parser files. At first I wanted to include them here, but despite things seemingly building fine it seems there's some additional implications with replacing %pure-parser with %define api.pure and %error-verbose with %define parse.error verbose that I can't judge because I'm not a bison expert.
• Possibly lots of platform specific stuff I've missed (didn't test with clang and we don't enable many warnings on Windows to begin with)

[Al2Klimov]

Note: This currently duplicates some of the efforts in #9411 until that is merged.

Suggestion:

1. Rebase #10609 on top of #9411
2. Change your base branch from master to compiler-warnings