O_BINARY versus O_TEXT.

O_BINARY and O_TEXT can be either different (mostly in Windowsy platforms,
and in some hybrids, and then they can be either effective or no-ops);
or equal (in UNIXy platforms, no-ops).  If they are no-ops, and especially
if one or both of them are zeros, one cannot even test for them (bit-and),
without introducing dead/non-sensical code.

[perl #121739]

Fix for Coverity perl5 CID 28948:
Logically dead code (DEADCODE)
dead_error_line: Execution cannot reach this statement mode[ix++] = 'b';