zshrc: do not enable lesspipe by default.

A recent post to full-disclosure[0] mentioned that this isn't very
secure.  Disable lesspipe by default.  Enhance the rless alias to
exclude LESSCLOSE as well, and add an sless function to be used only on
secure files.

[0] http://seclists.org/fulldisclosure/2014/Nov/74

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>