Add support for GNU-style command line options

[kofuk]

xclip supports only -option style option, but it's common for Unix command line tools to have
GNU style command line options (i.e. --option style.)

This PR adds support for GNU style command line option, keeping compatibility to Xrm* functions by removing
first hyphen if GNU style command line options are passed.

[hackerb9]

Your change looks reasonable and it is almost exactly what I was intending to add to close bug #1.

Have you thought about the behaviour in the very unlikely and definitely pathological, yet possible, case where a person is trying to read from a file named "--something--"? I don't see a simple and clean fix for that. Do you?

Personally, I think it is a bad idea to touch a lot of code to support GNU style long options as it could lead to complexity and breakage later. If you can't think of a succinct solution, my inclination is to merge your change and just add a BUG to the man page saying, "if your filename begins with two dashes, prefix it with ./ or xclip will try to read from the wrong file". I believe the real benefits to actual users outweighs the possible confusion to hypothetical ones.

Also, before this gets merged, I'd like @astrand to weigh in and give a thumbs up. Not only does this cause a new, albeit extremely minor, deficiency in xclip, I notice @astrand self-assigned fixing #1 and #16 back in 2016.

[hackerb9]

For code hygiene, I suggest declaring the counter variable within the for loop. E.g., for (int i = 1 ...

[kofuk]

I thought it should be compiled with ANSI C compiler. I'll use for (int i =... style if it allowed.

[astrand]

The common way of supporting a file name called --something is to treat everything after "--" as file names. See https://www.gnu.org/software/libc/manual/html_node/Using-Getopt.html .

One problem with support both -option and --option is that this is not a full GNU-style support: -option should mean the same thing as "-o -p -t -i -n". But it has been done before, see https://linux.die.net/man/1/vncviewer.

No easy solution so this problem, unfortunately, if we want to preserve backwards compatibility.

[kofuk]

Sorry for slow reply.

Treating options after -- as non-option argument seems a good way to cope with filenames starting with --.
Fortunately, xclip doesn't seem using a pattern of option that this solution can't treat -- ones like --option --filename-- where --option requires an argument.
(FYR, GNU-compatible option parser treats --filename-- as an argument for --something)

I think it's not a very big problem that -option is not interpreted as -o -p -t -i -o -n, as GNU C library also provides getopt_long_only(3) function.