Set a default PATH if PATH is empty or unset

mount(1) unsets PATH before calling mount.fuse. Therefore it's not set
in gocrpytfs either and daemonization fails if gocryptfs was not
executed via an absolute path.

mount.fuse handles this by leaving the execution of the helper to
/bin/sh. /bin/sh handles an empty PATH by searching a few default
locations.

This patch sets the PATH to a sane default if it's empty or unset.

main.go

@@ -126,6 +126,11 @@ func main() {
 		// On a 2-core machine, setting maxprocs to 4 gives 10% better performance
 		runtime.GOMAXPROCS(4)
 	}
+	// mount(1) unsets PATH. Since exec.Command does not handle this case, we set
+	// PATH to a default value if it's empty or unset.
+	if os.Getenv("PATH") == "" {
+		os.Setenv("PATH", "/usr/sbin:/usr/bin:/sbin:/bin")
+	}
 	var err error
 	// Parse all command-line options (i.e. arguments starting with "-")
 	// into "args". Path arguments are parsed below.