Properly handle symlinks (hardlinks and .so only files were already ok):

Properly handle symlinks (hardlinks and .so only files were already ok):
Use the file name of the symlink but the inode number of the file pointed to,
such that we get multiple mlinks records but not multiple mpages records.
Also make sure they do not point outside the tree we are processing.

Issue found by kili@ in desktop-file-edit(1), thanks!