manatee should support using the system postgres

[coyhile]

Outside Triton or Manta, users of manatee should allow easy use of the system PostgreSQL install (as from pkgsrc). The default sitter.json already allows one to set paths to initdb and postgres binaries.

I propose modifying the code that determines the paths to the postgres and initdb binaries such that if the following all hold, it should return the values of pgInitDbPath and postgresPath as specified in sitter.json rather than building the paths based on pgBaseDir:

• options.pgBaseDir is an empty string
• options.pgInitDbPath exists
• options.postgresPath exists