Input: Ensure clearTimer won't compute a lower nfds value than one of our opened fds

[NiLuJe]

By making sure in openInputDevice that the top member of the array also happens to be the highest fd number.

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Initial draft below:

Mention that nfds computations assume the inputfds array is sorted in increasing fd numbers

This doesn't necessarily hold true in practice (e.g., if one were to first open an fd during a directory walk with a few dir/file fds open...)

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This change is 

[NiLuJe]

Just so that I remember to fix that one day ;).

(Possibly by sorting the inputfds array on open/close, to keep the timer code simple and avoid having to walk it).

[NiLuJe]

Open random fds between input.open calls in Kobo (or basically any device with multiple input.open calls) to repro, e.g.,

diff --git a/frontend/device/kobo/device.lua b/frontend/device/kobo/device.lua
index e2df4a95a..dd2f94322 100644
--- a/frontend/device/kobo/device.lua
+++ b/frontend/device/kobo/device.lua
@@ -816,12 +816,16 @@ function Kobo:init()
     -- And then handle the extra shenanigans if necessary.
     self:initEventAdjustHooks()
 
+    local f = io.open("/dev/null", "r")
+    local f2 = io.open("/dev/null", "w")
     -- Various HW Buttons, Switches & Synthetic NTX events
     self.ntx_fd = self.input.open(self.ntx_dev)
     -- Dedicated Power Button input device (if any)
     if self.power_dev then
         self.input.open(self.power_dev)
     end
+    f:close()
+    f2:close()
     -- Touch panel
     self.input.open(self.touch_dev)
     -- NOTE: On devices with a gyro, there may be a dedicated input device outputting the raw accelerometer data

Then strace -fittvTyY -s128 -e trace=_newselect -p $(pidof reader.lua | sed -e 's/ / -p /g') to watch the horror unfold ;).