mirror/centos_mirror: Don't sync "versions"

Upstream CentOS mirrors actually maintain symlinks for major -> minor
versions, ie 6 -> 6.5 and 7 -> 7.0.1406, and they prune old releases
automatically (ie, 6.4 is actually just a readme file).
mirror/centos_mirror: Switch to safer bash paradigms

- Use global readonly variables
- Use function local variables
- Use flock to lock instead of touch / if tests
- Use functional programming paradigms
mirror/arch_mirror: Update for safer bash paradigms

- Use global readonly variables
- Use function local variables
- Use flock to lock instead of touch / if tests
- Use functional programming paradigms
mirror/arch_mirror: Remove --dry-run from testing
mirror/arch_mirror: Hardcode which rsync binary we want
mirror/centos_mirror: Hardcode rsync binary