Fix runtime errors when built with UBSan

nopen.c:37:15: runtime error: left shift of 1 by 31 places cannot be
 represented in type 'int'
nopen.c:39:18: runtime error: left shift of 1 by 31 places cannot be
 represented in type 'int'
userdat.c:251:111: runtime error: left shift of 1 by 31 places cannot be
 represented in type 'int'

There are lot of 1<<31 macro definitions in this repo, so I'm expecting a
lot more of this type of fix to be required to make UBSan happy.