Some description fixes

There are many more packages to fix, this is just a start.

Rules:
 * Don't repeat the package name (not always that easy...)
 * Start with capital letter
 * Don't end with full stop
 * Don't start with "The ..." or "A ..."

I've also added descriptions to some packages and rewritten others.

pkgs/tools/security/apg/default.nix

@@ -12,7 +12,7 @@ stdenv.mkDerivation rec {
   patches = [ ./apg.patch ];
 
   meta = {
-    description = "A tool set for random password generation.";
+    description = "Tools for random password generation";
     longDescription = ''
       APG (Automated Password Generator) is the tool set for random
       password generation.

pkgs/tools/security/clamav/default.nix

@@ -18,7 +18,7 @@ stdenv.mkDerivation rec {
 
   meta = with stdenv.lib; {
     homepage = http://www.clamav.net;
-    description = "an open source (GPL) antivirus engine designed for detecting Trojans, viruses, malware and other malicious threats.";
+    description = "Antivirus engine designed for detecting Trojans, viruses, malware and other malicious threats";
     license = licenses.gpl2;
     maintainers = [ maintainers.phreedom ];
     platforms = platforms.linux;