Replace colon escape sequences in GPG results (fixes #855)

Results from GPG are colon-separated (and colons in the values are replaced with an escape sequence). Example result line:

$ gpg --list-keys --with-colons
pub:-:4096:1:XXXXXXXXXXXXXXXX:XXXX-XX-XX:::-:BNVK \x3a) <hi@bnvk.me>::scESC:

So when parsing the results, after splitting the result string at the colons, the escape sequences needs to be converted back to colons.

fixes  #855