gpgdecrypt: improved error handling, return value

Script now exists with error status if input file is not a file or if gpg
throws an error.

Also, when writing the file name to stdout the script now prepends the
directory to which the file has been written.
opendec: first commit

opendec uses gpgdecrypt to decrypt a file, open it with gnome-open and then
remove it securely using srm.