Use explicit Timeout enum instead of magic constants

The FdReadableSet api was always intended to be converted to use Duration
instead of usec/msec once the ffi was removed. This lets us be explicit about
forever/infinite timeouts and removes the (small) chance of a collision between
u64::MAX and INFINITE.

I tried this out with `type Timeout = Option<Duration>` (only without the alias)
but was unhappy with easy it is to accidentally use `None` when you meant a
timeout of zero.

src/input_common.rs

@@ -5,7 +5,7 @@
     str2wcstring, write_loop, WSL,
 };
 use crate::env::{EnvStack, Environment};
-use crate::fd_readable_set::FdReadableSet;
+use crate::fd_readable_set::{FdReadableSet, Timeout};
 use crate::flog::{FloggableDebug, FLOG};
 use crate::fork_exec::flog_safe::FLOG_SAFE;
 use crate::global_safety::RelaxedAtomicBool;
@@ -334,9 +334,9 @@ fn readb(in_fd: RawFd, blocking: bool) -> ReadbResult {
 
         // Here's where we call select().
         let select_res = fdset.check_readable(if blocking {
-            FdReadableSet::kNoTimeout
+            Timeout::Forever
         } else {
-            0
+            Timeout::ZERO
         });
         if select_res < 0 {
             let err = errno::errno().0;