PostgreSQL 9.6, 10 with multiple discovery types

Follow-up to: !703 (merged)

Original issue: #498 (closed)

Linked version

$ brew link --force --overwrite postgresql@9.6
Linking /usr/local/Cellar/postgresql@9.6/9.6.13... 171 symlinks created

If you need to have this software first in your PATH instead consider running:
  echo 'export PATH="/usr/local/opt/postgresql@9.6/bin:$PATH"' >> ~/.zshrc

$ support/pg_bindir
/usr/local/bin

PostgreSQL 10 over 9.6

$ brew list | grep postgresql
postgresql@10
postgresql@9.6

$ ls -la /usr/local/bin/postgres
cannot access '/usr/local/bin/postgres': No such file or directory (os error 2)

$ support/pg_bindir
/usr/local/opt/postgresql@10/bin

PostgreSQL 9.6 only installed

$ brew list | grep postgresql
postgresql@9.6

$ support/pg_bindir
/usr/local/opt/postgresql@9.6/bin

No PostgreSQL installed

$ support/pg_bindir
Traceback (most recent call last):
support/pg_bindir:45:in `<main>': ERROR: Unable to determine PostgreSQL bin directory (RuntimeError)

Closes #526 (closed)