mnexec: mount sysfs in each container

The sysfs filesystem is [tagged][1] with a set of namespaces when mounted, taken
from the mounting process. Among other things, this controls which network
devices will show up in /sys/class/net and /sys/class/net/bonding_masters.

Without this change, mininet will not mount sysfs in a node. Attempting to
configure a bond interface in a node will only affect the parent namespace.

This change mounts a new sysfs filesystem in each node. To prevent this mount
from affecting the parent namespace the mount namespace is also unshared.

[1]: https://www.kernel.org/doc/Documentation/filesystems/sysfs-tagging.txt
Close the file when done with it.
Correct the comments of do_pingallfull() and do_iperfudp()
Close the file when done with it.
Correct the comments of do_pingallfull() and do_iperfudp()
Merge branch 'master' of github.com:yeasy/mininet
fixup: Simplify_chained_comparaison
fixup: useless_parenthesis
fixup: unresolved_references
Close the file when done with it.
Correct the comments of do_pingallfull() and do_iperfudp()
Merge branch 'master' of github.com:yeasy/mininet
Uploading MiniEdit 2.1.0.7.1

Many changes since master version.
- save/load topologies
- host, controller, switch and link properties
- link actions to set status up/down
Add notes about specific release branches.
Remove trailing spaces
Add #streamOptimized to fix .ovf for VirtualBox 4.3.2
fixes #261
If specified, add Branch to build dir name
Allow specification of branch to install
Allow -b to specify install or test branch
Fix typo/minor error.