Previously, repmgr would forcibly change the permissions on a data
directory to 0700. However from PostgreSQL 11, 0750 is also valid,
so that value should not be changed.
By checking the PID file in the same way pg_ctl does, we can be pretty
much certain whether the target data directory contains an active
PostgreSQL instance.
If --force-rewind is used in conjunction with "repmgr node rejoin",
any replication slots present on the source node will be copied too;
it's essential to remove these to prevent stale slots being extant
when the node starts up.
We do this at file system level *before* the server starts to minimize
the risk of any problems.
Addresses GitHub #334
