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>
# Conflicts:
# cmd/geth/main.go
# cmd/geth/usage.go
# cmd/utils/cmd.go
