webpmux: allow reading argument from a file

if a single text file name is supplied as argument
(e.g.: 'webpmux my_long_list_of_frames.txt'), the command
line arguments are actually parsed from this file.
Tokenizer will remove space, tabs, LF, CR, returns, etc.

+ changed ImgIoUtilReadFile() to return a null-terminated
data, for convenience.

+ misc clean-up in the code

BUG=webp:355

Change-Id: I76796305641d660933de5881763d723006712fa9
---

imageio/imageio_util.h

@@ -30,6 +30,9 @@ FILE* ImgIoUtilSetBinaryMode(FILE* file);
 // Allocates storage for entire file 'file_name' and returns contents and size
 // in 'data' and 'data_size'. Returns 1 on success, 0 otherwise. '*data' should
 // be deleted using free().
+// Note: for convenience, the data will be null-terminated with an extra byte
+// (not accounted for in *data_size), in case the file is text and intended
+// to be used as a C-string.
 // If 'file_name' is NULL or equal to "-", input is read from stdin by calling
 // the function ImgIoUtilReadFromStdin().
 int ImgIoUtilReadFile(const char* const file_name,