Preallocation is very slow on hdds that run btrfs. Give the
user the option to disable it. This greatly speeds up small file
operations but reduces the robustness against out-of-space errors.
Also add the option to the man page.
More info: https://github.com/rfjakob/gocryptfs/issues/63
Redirect stdout and stderr to /tmp/gocryptfs_paniclog.NNNNNN
instead of closing them so users have a chance to get the
backtrace on a panic.
This only applies if "-nosyslog" is NOT set. Panics will
go to terminal as usual if it is.
This makes sure all callers of parseMasterKey warn the user.
At the moment there is only one, but another one will be added
soon for forcing a password change when only the master key is
known.
For example, we cannot mount "/home/user/.cipher" at "/home/user"
because the mount will hide ".cipher" also for us.
Doing it anyway used to cause a nasty hang.
