Use _apt as our unprivileged user name

Some people want to standardize on it, and BSDs do it too, so let's
do the same.

Reported-by: Paul Wise <pabs@debian.org>

apt-pkg/contrib/fileutl.cc

@@ -2184,7 +2184,7 @@ bool DropPrivs()
    if(_config->FindB("Debug::NoDropPrivs", false) == true)
       return true;
 
-   const std::string nobody = _config->Find("APT::User::Nobody", "Debian-apt");
+   const std::string nobody = _config->Find("APT::User::Nobody", "_apt");
    struct passwd *pw = getpwnam(nobody.c_str());
    if (pw == NULL)
       return _error->Warning("No user %s, can not drop rights", nobody.c_str());

debian/apt.postinst

@@ -28,8 +28,8 @@ case "$1" in
 
         # add unprivileged user for the apt methods
         adduser --force-badname --system --no-create-home \
-            --quiet Debian-apt || true
-        chown -R Debian-apt:root \
+            --quiet _apt || true
+        chown -R _apt:root \
             /var/lib/apt/lists \
             /var/cache/apt/archives