Refactor common MIME magic matching code.

Put some methods in file_analysis::Manager that can perform the
matching process and return MIME type results.  Also helps to
centralize the management/re-use of a signature matcher object.

src/main.cc

@@ -933,6 +933,8 @@ int main(int argc, char** argv)
 
 		if ( rule_debug )
 			rule_matcher->PrintDebug();
+
+		file_mgr->InitMagic();
 		}
 
 	delete [] script_rule_files;