hardlink: rewrite files content comparison

hardlink: rewrite files content comparison

Example:

 # hardlink --ignore-time --dry-run /var/lib

Old version, based on memcmp(), nothing cached:

 Mode:           dry-run
 Files:          93453
 Linked:         36172 files
 Compared:       0 xattrs
 Compared:       25755615 files
 Saved:          80.11 MiB
 Duration:       204.557185 seconds

New version, --method=memcmp, 32 bytes cached:

 Mode:           dry-run
 Method:         memcmp
 Files:          93453
 Linked:         36172 files
 Compared:       0 xattrs
 Compared:       25755615 files
 Saved:          80.11 MiB
 Duration:       5.248426 seconds

New version, zero-copy (Linux crypto API), sha1, crc32 and sha256:

 Mode:           dry-run
 Method:         sha1
 Files:          93453
 Linked:         36172 files
 Compared:       0 xattrs
 Compared:       25755615 files
 Saved:          80.11 MiB
 Duration:       2.870973 seconds

 Mode:           dry-run
 Method:         crc32
 Files:          93453
 Linked:         36172 files
 Compared:       0 xattrs
 Compared:       25755615 files
 Saved:          80.11 MiB
 Duration:       2.582651 seconds

 Mode:           dry-run
 Method:         sha256
 Files:          93453
 Linked:         36172 files
 Compared:       0 xattrs
 Compared:       25755615 files
 Saved:          80.11 MiB
 Duration:       3.152825 seconds

The default is sha256 to be robust..

Signed-off-by: Karel Zak <kzak@redhat.com>