FileAnalysis: add custom libmagic database.

- It's derived from the magic database of libmagic 5.14, but with most
  everything not related to mime types removed.

- The custom database is always used by default for mime detection, but
  the more verbose file type detection will fall back on the default
  libmagic installation's database.  The result is: mime type strings
  are now guaranteed to be consistent across platforms, but the verbose
  file type descriptions are not.

- The custom database gets installed in $prefix/share/bro/magic, and
  should even be extensible if files with new patterns are added inside
  the directory.

- The search path for the mime magic database can be controlled via
  BROMAGIC environment variable.

- Remove mime_desc field from ftp.log.

- Stop using the mime/file type canonifier with unit tests.

- libmagic >= 5.04 is now a requirement.

magic/linux

@@ -0,0 +1,22 @@
+# See COPYING file in this directory for original libmagic copyright.
+#------------------------------------------------------------------------------
+# $File: linux,v 1.46 2013/01/06 21:26:48 christos Exp $
+# linux:  file(1) magic for Linux files
+#
+# Values for Linux/i386 binaries, from Daniel Quinlan <quinlan@yggdrasil.com>
+# The following basic Linux magic is useful for reference, but using
+# "long" magic is a better practice in order to avoid collisions.
+#
+# 2	leshort		100		Linux/i386
+# >0	leshort		0407		impure executable (OMAGIC)
+# >0	leshort		0410		pure executable (NMAGIC)
+# >0	leshort		0413		demand-paged executable (ZMAGIC)
+# >0	leshort		0314		demand-paged executable (QMAGIC)
+#
+
+# SYSLINUX boot logo files (from 'ppmtolss16' sources)
+# http://www.syslinux.org/wiki/index.php/SYSLINUX#Display_graphic_from_filename:
+# file extension .lss .16
+0	lelong	=0x1413f33d		SYSLINUX' LSS16 image data
+# syslinux-4.05/mime/image/x-lss16.xml
+!:mime image/x-lss16