Make POSTGRES_HOME override PATH.

[wdh]

At present POSTGRES_HOME is ineffective if a Postgres is found in PATH. This makes POSTGRES_HOME take precedence so PATH can always be overridden.

[nohuhu]

Looks good to me but this is somewhat significant behavior change so let's get other people chime in.

[TJC]

The PR here does more than just make POSTGRES_HOME have higher precedence -- it also makes the various @base_paths take precedence over $PATH too. That might cause surprising behaviour.

I wonder if it would make sense to bring things together, by getting rid of File::Which, and setting up all the paths in _search_paths().

That way we can maintain current precedence rules, apart from moving POSTGRES_HOME to the front.

ie. _search_paths() would return something like:
[ @ENV_POSTGRES_HOME, @ENV_PATH, @base_paths ]

[SysPete]

File::Which was added as part of 5fe92a8 to make finding binaries more reliable on win32 so I'd suggest keeping that.