Add command line options db-host and db-port

[timopollmeier]

What:
This adds the command line options --db-host and --db_port, which allow setting the host and port parameters for
connecting to the database.
This is currently only fully usable for local setups using UNIX domain sockets as authentication options for TCP/IP connections are still missing.

Why:
Changing the database host and/or port allows running gvmd with a non-default PostgreSQL server, for example for quickly testing different versions.

How:

• Run gvmd without any options for the default options
• Run gvmd with valid database connection options like --database gvmd --db-host "/var/run/postgresql/" --db-port 5432 for the default socket
• Run gvmd, each time replacing one of the option values above with an invalid one and check the gvmd log for error messages mentioning the invalid database name or socket paths.

Checklist:

• Tests
• CHANGELOG Entry