Try to inhibit USBMS on Kindles (#3957)

re: #3955

frontend/device/kindle/device.lua

@@ -83,10 +83,12 @@ function Kindle:setDateTime(year, month, day, hour, min, sec)
 end
 
 function Kindle:usbPlugIn()
-    -- NOTE: We do NOT support running in USBMS mode (we cannot, we live there).
-    --       And, AFAICT, we have no sane way of disabling USBMS mode without breaking either us or the framework,
-    --       c.f., https://github.com/koreader/koreader/issues/3220
-    --       That means shit will blow up in fun and interesting ways if someone actually tries that.
+    -- NOTE: We cannot support running in USBMS mode (we cannot, we live on the partition being exported!).
+    --       But since that's the default state of the Kindle system, we have to try to make nice...
+    --       To that end, we're currently SIGSTOPping volumd to inhibit the system's USBMS mode handling.
+    --       It's not perfect (f.g., if the system is setup for USBMS and not USBNet,
+    --       the frontlight will be turned off when plugged in), but it at least prevents users from completely
+    --       shooting themselves in the foot (c.f., https://github.com/koreader/koreader/issues/3220)!
     --       On the upside, we don't have to bother waking up the WM to show us the USBMS screen :D.
     -- NOTE: If the device is put in USBNet mode before we even start, everything's peachy, though :).
     self.charging_mode = true

platform/kindle/koreader.sh

@@ -42,6 +42,7 @@ fi
 
 # Keep track of what we do with pillow...
 export AWESOME_STOPPED="no"
+export VOLUMD_STOPPED="no"
 PILLOW_HARD_DISABLED="no"
 PILLOW_SOFT_DISABLED="no"
 PASSCODE_DISABLED="no"
@@ -243,6 +244,13 @@ if [ "${STOP_FRAMEWORK}" = "no" ] && [ "${INIT_TYPE}" = "sysv" ]; then
     killall -stop cvm
 fi
 
+# SIGSTOP volumd, to inhibit USBMS (sysv & upstart)
+if [ -e "/etc/init.d/volumd" ] || [ -e "/etc/upstart/volumd.conf" ]; then
+    logmsg "Stopping volumd . . ."
+    killall -stop volumd
+    VOLUMD_STOPPED="yes"
+fi
+
 # finally call reader
 logmsg "Starting KOReader . . ."
 # That's not necessary when using KPVBooklet ;).
@@ -292,6 +300,12 @@ if grep ${KOREADER_DIR}/fonts/linkfonts /proc/mounts >/dev/null 2>&1; then
     umount ${KOREADER_DIR}/fonts/linkfonts
 fi
 
+# Resume volumd, if need be
+if [ "${VOLUMD_STOPPED}" = "yes" ]; then
+    logmsg "Resuming volumd . . ."
+    killall -cont volumd
+fi
+
 # Resume cvm (only if we stopped it)
 if [ "${STOP_FRAMEWORK}" = "no" ] && [ "${INIT_TYPE}" = "sysv" ]; then
     logmsg "Resuming cvm . . ."