core: allow use of relative paths in configuration files on Windows
core: allow use of relative paths in configuration files on Windows

Fluent Bit has a feature that allows users to use a relative path
in the main configuration file. For example, if you have the
following files in the same directory:

    PS > ls C:\Program Files\td-agent\conf\
    parsers.conf fluent-bit.conf

Then you can just set "parsers.conf" to Parser_File as below:

    [SERVICE]
      Flush       5
      Daemon      Off
      Parser_File parsers.conf

Previously this feature did not work properly on Windows, due to
its path schema being incompatible with Unix. This fixes it.

Signed-off-by: Fujimoto Seiji <ef879192bb1e41d519c82cf76440dc8e9a530806@clear-code.com>

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