* backup/snaprsync: allow setting of program to use for summer; allow
setting of rsync options; allow settings to be set to empty strings on
the command line; do not fail if localprevious,rsums is missing.
* backup/snaprsync: allow setting of program to use for summer; allow
setting of rsync options; allow settings to be set to empty strings on
the command line; do not fail if localprevious,rsums is missing.