fuse: Set permissions from UNIX mode when available

If the GFileInfo has a UNIX mode available, use that for setting
permissions.  This prevents programs like vim from losing permissions
when saving a file.

https://bugzilla.gnome.org/show_bug.cgi?id=727996
Unify error conversion in http and dav backends

Have a single function which converts libsoup error codes to gio error
codes rather than one in each backend.

https://bugzilla.gnome.org/show_bug.cgi?id=536305
Report gvfs version when --version is used

https://bugzilla.gnome.org/show_bug.cgi?id=727132
client: Return NULL if set_display_name() fails

Return NULL as per the documentation rather than returning the original
file.  This prevents errors like the following, where "Permission
Denied" should be displayed:
$ gvfs-rename smb://localhost/tmp/cat dog
Rename successful. New uri: smb://localhost/tmp/cat

(gvfs-rename:1385): GLib-GObject-CRITICAL **: g_object_unref: assertion
'G_IS_OBJECT (object)' failed

https://bugzilla.gnome.org/show_bug.cgi?id=727007
run-in-tree.sh: Use sh rather than bash

We don't really on bash-specific features so use /bin/sh rather than
/bin/bash.

https://bugzilla.gnome.org/show_bug.cgi?id=726911