Bug#810145: pbuiler: find a way to mount /run/shm even if some other process is accessing it

[Jo Shields]

This is especially painful with the Jenkins Chroot plugin, which uses 
`pbuilder execute` to run things - a stray process from a failed CI test 
leads to gigs of "target is busy" messages in the build logs, which need 
manually killing.

This doesn't seem inordinately difficult to resolve - lsof can filter on 
BUILDPLACE, add a flag to kill stray processes, and just kill all the 
detected processes when that's turned on