[various] fix PID grab and allow for custom payload dirs

On NixOS, process names cant be reliably grabbed using pgrep or by
grepping ps. Instead we use the built-in BASH functionality for that,
despite what my previous comment may suggest about it's reliability.
Hopefully it doesn't break it.

The custom payload dirs thing was done to allow for potential packaging
of this application, in case it needs to be done.

startscreen.sh

@@ -83,6 +83,8 @@ function get_display_params {
 	TARGET_DISPLAY_MODE=$(echo $RESOLUTION/$DENSITY)
 }
 
+PAYLOAD_STORE_DIR=payload
+
 TARGET_CONNECTION_TIMEOUT=1
 TARGET_TMP_DIR=/data/local/tmp
 TARGET_SCRIPT1=$TARGET_TMP_DIR/scrcpy-payload1.sh
@@ -156,16 +158,17 @@ fi
 # use -S if you're edgy
 scrcpy --display $display -w -S -K &
 
-# Bash let me down so this is the alternative I have to work with
-SCRCPY_PID=$(pgrep scrcpy)
+# ???. Use the this one in case $! doesn't work??
+#SCRCPY_PID=$(pgrep scrcpy)
+SCRCPY_PID=$!
 
 #
 # Payload section starts here
 #
 
 # Execute the payload stream generator remotely (with precisely the parameters that it wants)
-adb push payload/stage1.sh $TARGET_SCRIPT1
-adb push payload/stage2.sh $TARGET_SCRIPT2
+adb push $PAYLOAD_STORE_DIR/stage1.sh $TARGET_SCRIPT1
+adb push $PAYLOAD_STORE_DIR/stage2.sh $TARGET_SCRIPT2
 adb shell chmod +x $TARGET_SCRIPT1
 adb shell chmod +x $TARGET_SCRIPT2