Multiple instance of freshclam startable

The user should be warned when starting a second instance of freshclam in case there's a usecase for doing so at all, otherwise the start should be allowed when specifying an option (e,.g. `-f`) on the command line.

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: clamav 0.98.6+dfsg-0ubuntu0.14.10.1
ProcVersionSignature: Error: [Errno 2] Datei oder Verzeichnis nicht gefunden: '/proc/version_signature'
Uname: Linux 3.19.2-031902-generic x86_64
ApportVersion: 2.14.7-0ubuntu8.2
Architecture: amd64
CurrentDesktop: Unity
Date: Sat Mar 28 10:07:13 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-01-26 (60 days ago)
InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: clamav
UpgradeStatus: No upgrade log present (probably fresh install)