Replace libmagic w/ Bro signatures for file MIME type identification.

Notable changes:

- libmagic is no longer used at all.  All MIME type detection is
  done through new Bro signatures, and there's no longer a means to get
  verbose file type descriptions (e.g. "PNG image data, 1435 x 170").
  The majority of the default file magic signatures are derived
  from the default magic database of libmagic ~5.17.

- File magic signatures consist of two new constructs in the
  signature rule parsing grammar: "file-magic" gives a regular
  expression to match against, and "file-mime" gives the MIME type
  string of content that matches the magic and an optional strength
  value for the match.

- Modified signature/rule syntax for identifiers: they can no longer
  start with a '-', which made for ambiguous syntax when doing negative
  strength values in "file-mime".  Also brought syntax for Bro script
  identifiers in line with reality (they can't start with numbers or
  include '-' at all).

- A new Built-In Function, "file_magic", can be used to get all
  file magic matches and their corresponding strength against a given
  chunk of data

- The second parameter of the "identify_data" Built-In Function
  can no longer be used to get verbose file type descriptions, though it
  can still be used to get the strongest matching file magic signature.

- The "file_transferred" event's "descr" parameter no longer
  contains verbose file type descriptions.

- The BROMAGIC environment variable no longer changes any behavior
  in Bro as magic databases are no longer used/installed.

- Reverted back to minimum requirement of CMake 2.6.3 from 2.8.0
  (it's back to being the same requirement as the Bro v2.2 release).
  The bump was to accomodate building libmagic as an external project,
  which is no longer needed.

Addresses BIT-1143.

src/rule-parse.y

@@ -34,6 +34,7 @@ static uint8_t mask_to_len(uint32_t mask)
 %token TOK_ENABLE
 %token TOK_EVAL
 %token TOK_EVENT
+%token TOK_MIME
 %token TOK_HEADER
 %token TOK_IDENT
 %token TOK_INT
@@ -61,9 +62,9 @@ static uint8_t mask_to_len(uint32_t mask)
 
 %type <str> TOK_STRING TOK_IDENT TOK_POLICY_SYMBOL TOK_PATTERN pattern string
 %type <val> TOK_INT TOK_TCP_STATE_SYM TOK_IP_OPTION_SYM TOK_COMP
-%type <val> integer ipoption_list tcpstate_list
+%type <val> integer ipoption_list tcpstate_list opt_strength
 %type <rule> rule
-%type <bl> TOK_BOOL
+%type <bl> TOK_BOOL opt_negate
 %type <hdr_test> hdr_expr
 %type <range> range rangeopt
 %type <vallist> value_list
@@ -186,6 +187,9 @@ rule_attr:
 	|	TOK_EVENT string
 			{ current_rule->AddAction(new RuleActionEvent($2)); }
 
+	|	TOK_MIME string opt_strength
+			{ current_rule->AddAction(new RuleActionMIME($2, $3)); }
+
 	|	TOK_ENABLE TOK_STRING
 			{ current_rule->AddAction(new RuleActionEnable($2)); }
 
@@ -359,6 +363,20 @@ integer:
 			{ $$ = id_to_uint($1); }
 	;
 
+opt_negate:
+		'-'
+			{ $$ = true; }
+	|
+			{ $$ = false; }
+	;
+
+opt_strength:
+		',' opt_negate TOK_INT
+			{ $$ = $2 ? -$3 : $3; }
+	|
+			{ $$ = 0; }
+	;
+
 string:
 		TOK_STRING
 			{ $$ = $1; }