yum/apt mirror support in up2date

as of up2date 4.3, up2date supports a list of mirrors for
each channel. 

The configuration for mirrors is spread out a bit for
flexibility, but should be easy enough.

The first thing to enable use of mirrors is to add a
"yum-mirror" or "apt-mirror" config line in
/etc/sysconfig/rhn/sources. This line is of
the format:

yum-mirror  channel-label  uri
  or
apt-mirror  channel-label uri

The uri is a location where a mirror list can be
found. This can be a local file:// url or a http
url. This file will be fetched, and parsed. The format
is one url per line, with the url being equilivent to
the base url in the sources file:

for example, a mirrors list for fedora-core-1 might look like:

http://rpmfind.net/linux/fedora/core/1/i386/os/
http://distro.ibiblio.org/pub/linux/distributions/fedora/linux/core/1/i386/os/
http://mirror.eas.muohio.edu/fedora/linux/core/1/i386/os/
http://mirror.hiwaay.net/redhat/fedora/linux/core/1/i386/os/
http://ftp.dulug.duke.edu/pub/fedora/linux/core/1/i386/os/
http://redhat.secsup.org/fedora/core/1/i386/os/

By default, up2date will fetch this file each time it is
ran, and update it. 

If the user wants to limit the list of mirrors, they
can create a file in /etc/sysconfig/rhn/mirrors/ with
the name of the channel as the filename. For example,
/etc/sysconfig/rhn/mirrors/fedora-core-1 might contain:

http://distro.ibiblio.org/pub/linux/distributions/fedora/linux/core/1/i386/os/
http://ftp.dulug.duke.edu/pub/fedora/linux/core/1/i386/os/

up2date will parse this file, and randomly pick one of the
mirrors in it to use for the duration of the up2date sesssion. 
Note that if the /etc/sysconfig/rhn/mirrors/CHANNELNAME file
exists, up2date will use it, and will not fetch the mirror
list from the uri listed in /etc/sysconfig/rhn/sources for
the "yum-mirror" type

Note: if the yum-mirror line doesnt exist in the config
file for a given channel, no mirrors will be used and
the url specified for that channel will be used instead.