Trust but verify 'scutil' to return ComputerName

Apparently, it is possible to set up a Mac such that
`scutil --get ComputerName` hasn't been set.

This change checks if that fails and falls back to the original mechanism.

Closes #2155
Closes #2183
`scutil` should continue being used without absolute path. Also, on #2183 they use `$(hostname -s)', is that better than using $HOST?