add support for the snapshot manager flag that creates a revert boot menu entry

[trgill]

After support for the "-b" (boot menu entry) is added - also add "-r" (revert boot menu entry)

# snapm snapset create -br --size-policy 100%USED backup / /home /var
SnapsetName:      backup
Sources:          /, /home, /var
NrSnapshots:      3
Time:             2024-12-05 19:10:44
UUID:             d3f5e3cd-a383-5dba-b597-9134a2c426e9
Status:           Active
Autoactivate:     yes
Bootable:         yes
BootEntries:
  SnapshotEntry:  f574a20
  RevertEntry:    f428f9f