Add build-time distribution detection to Makefile

When using the same policy on several hosts with different Linux distributions
the DISTRO variable can't be defined in the build.conf of the build directory
because this file is tracked with git.

Instead of maintaining a local patch per host to define DISTRO in build.conf
or of building using "make DISTRO=...", this commit introduces a DISTRO_DETECT
boolean in build.conf which automatically fills DISTRO with the name of the
distro of the building host.

DISTRO_DETECT definition is not copied in the installed build.conf file
(/usr/share/selinux/refpolicy/include/build.conf) because this file would have
the DISTRO variable set.
Customize build and README
Make usage of deprecated interfaces fatal
Make policy/modules/contrib a local submodule
Fix /var/run/syslog-ng.ctl label
Debian patch 3: Make default and root mcs/seusers unconfined
Debian patch 4: Add additional interfaces and a boolean switch to access unconfined
Debian patch 8: Add pythonsupport policy
Debian patch 9: Add a new boolean to grant users access to dosfs_t.
Debian patch 10: Allow users read access to /var/lib/apt/lists
Debian patch 14: Debian nagios file locations and additional policy

This addresses distribution specific file locations, allows the nagios mail
plugin to run sudo for the purpose of directly searching postfix spools (more
policy needed for other MTAs), allows nrpe to create tmp files and do all
networking stuff, and to talk to database servers.
Debian patch 15: Move /dev/xconsole from xserver policy to logging.pp

The patch moves the xconsole policy to logging.pp from the xserver
policy.  It's more about logging than the X server and there are a lot of
systems which have syslogd configured to write to /dev/xconsole but that have
no X server installed.
Debian patch 16: Debian specific changes for init_t
Debian patch 19: gpg.{fc,if,te}: Fixes so that gpg-agent starts up (also when
 launched by ssh-agent). Add interfaces to access to transition to
 gpg_agent_t. unprivuser.te, ssh.if: Make use of the new interfaces.
 userdomain.fc: Label user's .gnupg
Debian patch 23: Watchdog policy: Allow watchdog_t to also read watchdog files
Debian patch 24: Use filesystem transitions for hugetlbfs_t
Debian patch 25: files.if: Allow management of symlinks under /mnt; also add
 interface files_manage_pid_dirs which is not used in standard
 policy for local system administrator's use
Debian patch 26: tmpreaper policy: Some debian customisations, also make tmpreaper
 policy MCS-aware (add interface in mcs.if for that)
Debian patch 27: Refinements for MCS policy
Debian patch 28: add getattr access to selinux_validate_context