Let CiviCRM accept unix domain sockets for the database URL.
Currently the database address input of the onboarding page expects an address of the format of $host:$port number.
The issue is that some databases may be set up instead to communicate through a unix domain socket.
A solution would be to allow for additional address parsing that would recognise the use of a socket instead of a port number.
This address format is supported by mysqli_connect already, so we just need to adapt our handling of the input field. The solution would allow the user to specify the address something like unix:///var/run/mysql-socket.