Parse hostname from /proc/sys/kernel/hostname for Unix

[DissectBot]

Sometimes we have /proc available (local targets or acquires with proc collected). We should add /proc/sys/kernel/hostname to the path of files to check for the hostname for Unix targets.

https://github.com/fox-it/dissect.target/blob/a314d250f7e1b40adc841fb526cb5bd32de6159d/dissect/target/plugins/os/unix/_os.py#L144(https://github.com/fox-it/dissect.target/blob/a314d250f7e1b40adc841fb526cb5bd32de6159d/dissect/target/plugins/os/unix/_os.py#L144|smart-link)

[CinisSec]

Tried to implement this with following pull request #704