pgfs: cross-platform support (or not)

[erichanson]

• use build constraints to only build pgfs on supported platforms
• use pgfs_unsupported.go on unsupported platforms (mac etc)
• add PGFS.Enabled variable, defaulting to false in dist configs

[micburks]

Works after these changes

[erichanson]

Works on Mac w/o pgfs support, props to @micburks for figuring out no-op pattern.