Add an UnescapeAndSplit function

In many cases, where we accept (usually from the command line) a list of
parameters, we remove the use of the separator as an component of any of
the elements.

This patch adds a new function that can split strings of the form
'a,b\,c,d' into ['a', 'b,c', 'd'], with proper un-escaping of
double-backslashes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>