Use lengthof instead of sizeof(A)/sizeof(A[0])

The latter form can accept pointers, but the template lengthof cannot.
Switch PHYSFSX_fgets to auto-sized variant where possible
Mark PHYSFSX_find* functions nonnull
Remove unused flag OF_HARMLESS
Fix array overstep on invalid PCX error number
Add missing va_end calls
Remove unused return value of gr_*printf / gr_*string