Support BSD

[klemensn]

• Detect non-Mac and non-Windows as Unix to support BSDs next to Linux
• Extend Linux specific Thunderbird support to UNIX to include BSDs
• Enable progressbar stylesheets on BSDs just like on Linux
• Right-align positive buttons on BSDs as well

The first commit is required to unbreak the build on OpenBSD. The others are optional fixes to improve user experience and code consistency.

I am now running qdigidoc4-4.2.11 with libdigidocpp-3.14.8 on OpenBSD/amd64 7.1.

I have successfully encrypted and decrypted a file on OpenBSD and Linux (NixOS 21.11), respectively.

I can view my card's details and inspect the email forwarding setting.

Signing files, however, causes a crash due to a use-after-free() bug; this happens with digidoc-tool create --file=./hello.txt hello.asice from libdigidocpp as well as the graphical qdigidoc4 interface.

On Linux, signing files also fails with qdigidoc4 showing a generic error message. I have not looked into this (yet).