metarestore: Fix memory leak

Fix memory leak (it was found by valgrind when running
test_metadata_recovery) by refactoring merger's interface
to use std::string instead of c-style strings.

Change-Id: I5bb85a4ca99b69a39a8eb8158a5d57334cda3b39
mount: Fix getting xattrs for special inodes

.masterinfo file has to be readable by unpriviledged users in order to be
able to use mfstools. There was a bug in mfs_getxattr handler which caused
EPERM to be returned when kernel asked for ACLs for this inode. This commit
fixes this issue by making mount return empty xattrs for all special inodes.

Change-Id: Iad57757ac43b60d0a58c1c0892379a89a129b654