Expand `get_mtime` support for accesses

This issue track the current mtime support across existing accesses:

• AVIO (AVIO does not expose mtime yet)
• Concat
• Data
• DSM
• Emjsfile
• File (!2195 (merged))
• FTP
• HTTP
• Imem
• MTP
• NFS
• Samba
• SDP
• SFTP
• SMB2
• UNC
• VCD
• VDR

Disabled check-boxes indicate that the access likely cannot provide the mtime value.