Improve READ handling on Welcome Screen

[tanabi]

READ has been disabled on the Welcome screen. See #675 and #674 for the entire lineage of this issue.

This should be improved in some fashion. Perhaps the addition of READ_DESCR to read from a specific descriptor, or maybe when entering IN_READ in interp.c we branch on the detection of being on the welcome screen and handle the 'blocking' different. Right now, blocking (i.e. accepting read) is intimately connected to a player object so it can't work with #-1 without significant change. We could add a concept of a blocked descriptor and automatically use that when there's not a player available for instance.