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
---

man/webpmux.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH WEBPMUX 1 "November 10, 2016"
+.TH WEBPMUX 1 "December 1, 2017"
 .SH NAME
 webpmux \- create animated WebP files from non\-animated WebP images, extract
 frames from animated WebP images, and manage XMP/EXIF metadata and ICC profile.
@@ -48,6 +48,8 @@ frames from animated WebP images, and manage XMP/EXIF metadata and ICC profile.
 .B webpmux [\-h|\-help]
 .br
 .B webpmux \-version
+.br
+.B webpmux argument_file_name
 .SH DESCRIPTION
 This manual page documents the
 .B webpmux
@@ -55,6 +57,9 @@ command.
 .PP
 \fBwebpmux\fP can be used to create/extract from animated WebP files, as well as
 to add/extract/strip XMP/EXIF metadata and ICC profile.
+If a single file name (not starting with the character '\-') is supplied as
+the argument, the command line argument are actually tokenized from this file.
+This allows for easy scripting or using large number of arguments.
 .SH OPTIONS
 .SS GET_OPTIONS (\-get):
 .TP