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/utils/flags.go

@@ -113,6 +113,10 @@ var (
 		Name:  "datadir.ancient",
 		Usage: "Data directory for ancient chain segments (default = inside chaindata)",
 	}
+	MinFreeDiskSpaceFlag = DirectoryFlag{
+		Name:  "datadir.minfreedisk",
+		Usage: "Minimum free disk space in MB, once reached triggers auto shut down (default = --cache.gc converted to MB, 0 = disabled)",
+	}
 	KeyStoreDirFlag = DirectoryFlag{
 		Name:  "keystore",
 		Usage: "Directory for the keystore (default = inside the datadir)",