feat: Samba friendly snapshot name

[kimata]

Renaming a snapshot to make it easier to handle with Samba's vfs_shadow_copy2.

After applying the above changes, the following settings in smb.conf should work.

     shadow:format       = _UTC_%Y-%m-%d_%H:%M:%S                                                                                                                                                          
     shadow:snapprefix   = ^autosnap_\(yearly\)\{0,1\}\(monthly\)\{0,1\}\(weekly\)\{0,1\}\(daily\)\{0,1\}\(hourly\)\{0,1\}\(frequently\)\{0,1\}                                                            
     shadow:delimiter    = _UTC_                                                                                                                                                                           
     shadow:localtime    = no

[stumpylog]

This would be great to have included in sanoid. I use ZFS and share out some datasets using Samba.

I wonder if there needs to be some sort of rename of existing snapshots? To facilitate the pruning of old named snapshots?

[sporkus]

This and removing the colons would help windows users. vfs_shadow_copy2 does not allow regex/wildcard after the time string.

[natankeddem]

What would it take to get this feature actually implemented? I am going to just spitball some ideas:

• Make it enabled with an argument not a blanket change for everyone to preserve backward compatibility
• remove colons from date since that seems to cause some people issues as well (although I haven't experienced this tbh)
• ???