Document requirement to disable mount namespace separation with recent versions of SuperSU

[rutsky]

New versions of SuperSU uses mount namespace separation, so even if Mountie manages to mount device, it will not be visible for other application.
Mount namespace separation can be disabled in SuperSU settings.

Mountie should at least warn users about such possible issue and provide information how to workaround it (by disabling namespace separation in SuperSU).

[rutsky]

According to documentation looks like it's not required to disable namespace separation in latest versions SuperSU globally, instead we can start su with --mount-master option.

But first it is required to check that used su command is provided by SuperSU, and SuperSU has specific version.
According to SuperSU changelog:

• mount namespace separation introduced in v1.50
• --mount-master introduced in v1.93

So for versions prior v1.50 nothing should be done, for versions v1.50 <= x < v1.93 change in SuperSU global setting should be adviced, and for versions from v1.93 --mount-master option should be used.

su version checking is not implemented in embedded version of RootTools, but implemented in newer version of RootTools/RootShell: https://github.com/Stericson/RootShell/blob/92c2a85e93d9a5e966fddd76754abb12b97f8536/src/com/stericson/RootShell/execution/Shell.java#L421

[rutsky]

I opened feature request to support --mount-master in RootShell: Stericson/RootShell#7.