metadata: Get tree from udev via dbus

We don't want to have dependency on udev in client part of gvfs due
to app sandboxing. Thus move the code into the metadata daemon and
get tree name via dbus.

Common code to get metadata proxy has to be moved in metatree code.

Mentioned changes caused that unistd.h is also included in
meta-get-tree.c and cause symbol redeclaration error. Variable
"pause" has to be renamed to avoid this conflict.