cmd: handle spaces in directory names in del /s

[Dan Kegel]

Adds quotes as suggested by Jason Edmeades.
Fixes bug 26885.
Passes winetestbot.

 builtins.c                  |    5 ++++-
 tests/test_builtins.cmd     |   20 ++++++++++++++++++++
 tests/test_builtins.cmd.exp |    1 +
 3 files changed, 25 insertions(+), 1 deletion(-)
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