fix several gtags related issues: mission update_gtags.cmd & lack of execution permission

[skywind3000]

After the 39dc3ee commit, gtags generation will require a update_gtags.sh or update_gtags.cmd script to execute the actual gtags command.

But:

1. "update_gtags.sh" on linux platform lacks execution permission
2. "update_gtags.cmd" is missing on Windows platform.

And gtags_cscope became unusable on both Linux and Windows due to these two issues.

This PR fixed:

• update_gtags.sh lost execution permission #335
• ERROR: Invalid value for argument cmd and update_gtags.sh is not executable #322
• use gtags failed #332
• Cannot generate the GTAGS at windows platform #327

[skywind3000]

@ludovicchabant , maybe you are busy, but do you have several minutes to review this ?
because gtags_cscope is not usable on both Linux and Windows right now.

and many users complaint this in the issues.

[ludovicchabant]

Thanks for this! Yes, I'm slow with dealing with PRs and issues -- apologies for that.

What are the pros/cons between your version of update_gtags.cmd and the one that came from a previous PR? (see 4108ba2 )

[skywind3000]

1. My version is directly translated from update_gtags.sh, but his version is not.
2. My version supports FILE_LIST_CMD (like update_gtags.sh), but his version doesn't
3. The arguments parsing routine of his version is incorrect, he can only accept --incremental as the only one possible argument for gtags, but my version accepts arbitrary arguments for gtags.

[ludovicchabant]

That's the impression I got from a quick diff, thanks. I updated the file with your version, included the Powershell support fix, but stripped the changeset that fixed the file permissions since this is already in. See 1337b18. Thanks a lot!

[skywind3000]

thanks