Windows: Clean up text file packaging.

Convert textify.sh to PowerShell. Use PowerShell's built-in line ending
conversion so that we don't depend on unix2dos.

Only copy the help toc and text files to the staging directory.

Add PowerShell to the Developer's Guide. Fixup some other content.

(asn1/Makefile.inc.nmake contains a call to u2d. Hopefully that's not
a problem.)

Change-Id: I61a92aa54820d01015abb9ffa65815558ae31c71
Reviewed-on: https://code.wireshark.org/review/3487
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>
Petri-Dish: Graham Bloice <graham.bloice@trihedral.com>