more mounts in mock

[Mike McLean]

Attached are a couple of patches that expand the mounts created in the 
chroot by mock. These are mounts that we've used for builds within Red 
Hat for years and some packages need them to compile properly.

more_mounts.patch is the larger patch, it refactors _mount() so that the 
mounts to be created are specified in a list and looped over.  I've also 
changed to the unmounting code to make it more paranoid. In order to 
allow these mounts, I had to make some changes to mock-helper.

bind_dev.patch builds on the the previous patch and provides an option 
to have /dev bind mounted in the chroot (instead of the skeletal /dev 
that mock sets up). The patch makes this an option with the original 
behavior as the default.