Cygwin: mkvers: fix a bug in sed statement

Cygwin: mkvers: fix a bug in sed statement

While reformatting the script, backticks `` were replaced with
brackets $().  This in turn invalidated the \\( ... \\) expressions in the
sed script because backslash resolution in $() works differently from
backslash resolution in ``.  Only a single backslash is valid now.

While at it, fix up the uname(2) date representation when building a
snapshot.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>