add an option for more verbose output (than --verbose) to mount.cifs

According to `man mount.cifs` debugging is done with the `--verbose` option and inspecting files in `/proc`. In some cases `--verbose` isn't nearly sufficient, so that a `--very-verbose`/`-vv` or `--debug` option would be useful. An example is the following output:

    $ sudo mount.cifs //192.168.178.130/data_extension /mnt/data_extension/ --verbose -o uid=1000,gid=1000,rw,user=user
    Password for user@//192.168.178.130/data_extension: ************
    mount.cifs kernel mount options: ip=192.168.178.130,unc=\\192.168.178.130\data_extension,uid=1000,gid=1000,user=user,pass=********
    Retrying with upper case share name
    mount.cifs kernel mount options: ip=192.168.178.130,unc=\\192.168.178.130\DATA_EXTENSION,uid=1000,gid=1000,user=user,pass=********
    mount error(6): No such device or address
    Refer to the mount.cifs(8) manual page (e.g. man mount.cifs)

It's possible to give way more feedback in this case of failure (is the remote host reachable?, what shares are available?). `testparm` on the remote machine shows that the share is available and `ping` succeeds, so cifs internals causing the failure need to be exposed here to give any clue what might have gone wrong.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: cifs-utils 2:6.0-1ubuntu2
Uname: Linux 4.0.0-040000-generic x86_64
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Apr 23 10:15:31 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-02 (20 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: cifs-utils
UpgradeStatus: No upgrade log present (probably fresh install)