Update Debian packaging

The main goal is to reduce the difference with the "official" Debian packages. I have split the various changes in several commits. The changes with the "official" Debian packages are now quite minimal:

 - `debian/changelog` (explained why in the appropriate commit)
 - `debian/control` because of the renaming from `rancid` to `rancid-git`
 - `debian/rules` for the same reason, plus I removed the renaming of `par` to `rancid-par`
 - no patches

Here is the debdiff with the previous versions:

For `rancid-git`:

```
Files in second .deb but not in first
-------------------------------------
-rw-r--r--  root/root   /etc/cron.d/rancid-git
-rwxr-xr-x  root/root   /var/lib/rancid/downreport
-rwxr-xr-x  root/root   /var/lib/rancid/getipacctg
-rwxr-xr-x  root/root   /var/lib/rancid/rtrfilter

Files in first .deb but not in second
-------------------------------------
-rw-r--r--  root/root   /usr/share/doc/rancid-git/examples/cron.example
-rwxr-xr-x  root/root   /usr/lib/rancid/bin/downreport
-rwxr-xr-x  root/root   /usr/lib/rancid/bin/getipacctg
-rwxr-xr-x  root/root   /usr/lib/rancid/bin/rtrfilter

Control files: lines which differ (wdiff format)
------------------------------------------------
[-Conflicts:-]
{+Breaks:+} rancid-core (<< 2.3.5), rancid-util (<< 2.3.5)
{+Conflicts: par, rancid+}
Depends: libc6 (>= 2.3.4), debconf (>= 0.5) | debconf-2.0, [-expect,-] perl, [-git-] {+expect, cvs+} | subversion | [-cvs,-] {+git,+} passwd, openssh-client | ssh, [-iputils-ping-] {+inetutils-ping+} | ping, [-telnet,-] adduser, [-exim4-] {+libperl4-corelibs-perl+} | [-mail-transport-agent-] {+perl (<< 5.12.3-7)+}
Description: Really Awesome New Cisco [-config-] {+confIg+} Differ
 software and hardware (cards, serial numbers, etc), using [-CVS-] {+CVS,+}
{+ Subversion+} or
 [-Subversion.-] {+Git.+}
Installed-Size: [-1290-] {+1295+}
Provides: {+rancid,+} rancid-core, rancid-util
Version: [-2.3.9-1-] {+2.3.9-0+}
```

And for `rancid-cgi`:

```
Control files: lines which differ (wdiff format)
------------------------------------------------
Depends: {+perl,+} rancid-git, liblockfile-simple-perl
Description: {+looking glass+} CGI for rancid
Installed-Size: [-142-] {+137+}
Version: [-2.3.9-1-] {+2.3.9-0+}
```