SYSV: change instances of simplename to simplefile

This is to better match the schema since simplefile already exists. The
previous simplename attribute would fail validation. Since pkgmgr
already helpfully constructs url for you if simplefile exists, the tool
no longer needs to do the concatenation itself.

Given the low usage rate of SYSV.py and that the original functionality
was introduced in a late 1.3 release, changing the name w/o providing
backwards compatiblity seems reasonable.
doc: Update SYSV datastream format docs
SYSV: add a migration tool to change simplename attributes to simplefile
Merge branch 'SYSV-use-simplefile' of https://github.com/fennm/bcfg2 into maint