update-checksums.sh: Do not back up PKGBUILD

Use git for that.
update-checksums.sh: Wipe sed script before writing to it
update-checksums.sh: Exit if sed fails
Add btsync-autoconfig@.service, don't run btsync-wrapper

The overall aim is to extract the config generator scripts to their own
add-on package.
Rename btsync-wrapper.sh to btsync-autoconfig.sh
Do not execute btsync in btsync-autoconfig.sh
btsync-autoconfig.sh: Accept a single pos-param instead of option
btsync-autoconfig.sh: Improve output
Invoke shell scripts as arguments to bash
Make btsync.config without btsync-makeconfig.sh
Install btsync-autoconfig
Group btsync-autoconfig stuff at the end of PKGBUILD
Update checksums
Prepare for breaking config generator out as addon

- Eliminate btsync-wrapper, use a hook-on systemd service
  (btsync-autoconfig@.service) instead
- Do not use btsync-makeconfig.sh to create /etc/btsync.conf
Move autoconfig files to their own package
Add gitignore to autoconfig
Add PKGBUILD and .install to autoconfig
Fix script paths
En/disable autoconfig service on install/remove
Update checksums