Uw standby monitors

Add `gproc:monitor(Name, Type) -> Ref`, where `Type :: info | standby | follow`

`gproc:monitor(Name, info)` is equivalent to `gproc:monitor(Key)`.

Using `standby`, the registered name is inherited by the monitoring process (the first in line) if the previous owner dies. Value and properties (including monitors) are also inherited, but monitors do not survive an explicit `unreg()`.

`gproc:monitor(Name, follow)` reports the same events as `info`, but also `registered`, as the monitor survives unregistrations, and can also be set before `Name` is registered.