Fix get_bloat for Windows and use PGPASSFILE

[marcohald]

Thisis a recreation of https://github.com/Checkmk/checkmk/pull/560

Bug reports

Please include:

• Your operating system name and version Windows Server 2019
• Detailed steps to reproduce the bug

1. use more than 3 Postgres DBs Then the get_bloat crashes with "The command line is too long."

2. Set a System Environment Variable for the PGPASSFILE It get's overwritten by an empty String

Proposed changes

This commit fixes two issue.

1. The get_bloat executes psql for each database. This is necessary because otherwise it exceeds the maximum 8191 characters limit of windows https://learn.microsoft.com/en-us/troubleshoot/windows-client/shell-experience/command-line-string-limitation Which triggers the error "The command line is too long." I observed it with 3 databases

2. The PGPASSFILE Env varaible is only overwritten by the instance pg_passfile when it is not empty. In this way it is possible to use a system wide PGPASSFILE evnironment variable.