Remove PackagesOnHold from xbps.conf; use xbps-pkgdb -m hold|unhold instead.

To put a package on hold mode:
	$ xbps-pkgdb -m hold foo

To unhold the package:
	$ xbps-pkgdb -m unhold foo

To list packages on hold mode:
	$ xbps-query -H

This also close #12 from github.