cmd/geth: graceful shutdown if disk is full (#22103)

Adding warnings of free disk space left and graceful shutdown when there is not enough space left.
This also adds a flag datadir.minfreedisk which can be used to set the trigger for low disk space, and setting it to zero disables the check. 

Co-authored-by: Martin Holst Swende <martin@swende.se>
Co-authored-by: Felix Lange <fjl@twurst.com>

cmd/geth/usage.go

@@ -36,6 +36,7 @@ var AppHelpFlagGroups = []flags.FlagGroup{
 			configFileFlag,
 			utils.DataDirFlag,
 			utils.AncientFlag,
+			utils.MinFreeDiskSpaceFlag,
 			utils.KeyStoreDirFlag,
 			utils.USBFlag,
 			utils.SmartCardDaemonPathFlag,