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.

testing/btest/Baseline/scripts.base.frameworks.file-analysis.http.partial-content/a.out

@@ -1,7 +1,7 @@
 FILE_NEW
 7gZBKVUgy4l, 0, 0
-FILE_TYPE
-mime type is set
+MIME_TYPE
+application/pdf
 FILE_OVER_NEW_CONNECTION
 FILE_STATE_REMOVE
 7gZBKVUgy4l, 555523, 0

testing/btest/Baseline/scripts.base.frameworks.file-analysis.http.partial-content/b.out

@@ -1,7 +1,7 @@
 FILE_NEW
 oDwT1BbzjM1, 0, 0
-FILE_TYPE
-mime type is set
+MIME_TYPE
+application/x-dosexec
 FILE_STATE_REMOVE
 oDwT1BbzjM1, 1022920, 0
 [orig_h=192.168.72.14, orig_p=3254/tcp, resp_h=65.54.95.206, resp_p=80/tcp]
@@ -9,8 +9,8 @@ total bytes: 1022920
 source: HTTP
 FILE_NEW
 oDwT1BbzjM1, 0, 0
-FILE_TYPE
-mime type is set
+MIME_TYPE
+application/octet-stream
 FILE_TIMEOUT
 FILE_STATE_REMOVE
 oDwT1BbzjM1, 206024, 0

testing/btest/Baseline/scripts.base.frameworks.file-analysis.http.partial-content/c.out

@@ -1,7 +1,7 @@
 FILE_NEW
 uHS14uhRKGe, 0, 0
-FILE_TYPE
-mime type is set
+MIME_TYPE
+application/octet-stream
 FILE_OVER_NEW_CONNECTION
 FILE_STATE_REMOVE
 uHS14uhRKGe, 498702, 0