Adding event mask validation for ModifyWaitEvent

Hi,

Please find attached a patch implementing an old FIXME comment [1]
about validating the event mask in ModifyWaitEvent(). To prevent callers
of ModifyWaitEvent() from passing invalid flags that can be silently
accepted, I propose adding two checks:

 1. Prevent setting socket wait flags on an event that has no socket,
mirroring line 600 of AddWaitEventToSet() [2].

 2. Prevent promoting a non-latch event to a latch event
through ModifyWaitEvent(). The setup for latch events is handled in
AddWaitEventToSet(), but not supported in ModifyWaitEvent(). For example,
AddWaitEventToSet() enforces "cannot wait on more than one latch" [3] per
set, registering the latch pointer [4], and handles platform-specific latch
behavior [5]. We do still allow the behavior documented in the comment for
ModifyWaitEvent() [6] about setting a latch to NULL to disable it, and
enabling again as a latch later.

[1] The commit adding this code dates back to 2016, added in
https://github.com/postgres/postgres/commit/98a64d0bd71#diff-6e542ba2eb1d83ef90e65cdc0912b51a295184701c7e3bd236937c43c4cac4b9R704
[2]
https://github.com/postgres/postgres/blob/master/src/backend/storage/ipc/waiteventset.c#L600
[3]
https://github.com/postgres/postgres/blob/master/src/backend/storage/ipc/waiteventset.c#L588-L589
[4]
https://github.com/postgres/postgres/blob/master/src/backend/storage/ipc/waiteventset.c#L614-L615
[5]
https://github.com/postgres/postgres/blob/master/src/backend/storage/ipc/waiteventset.c#L616-L619
[6]
https://github.com/postgres/postgres/blob/master/src/backend/storage/ipc/waiteventset.c#L650-L651