MB-7390: Do not allow backslashes in Mochiweb serve_file
MB-7390: Do not allow backslashes in Mochiweb serve_file

On Windows, Mochiweb does not correctly safeguard against backslash
characters and so arbitrary files can be accessed. While we wait for
patch to make it's way into Mochiweb, this change closes the hole in
a few obvious places in ns_server.

Change-Id: Iab8d8320ac11cedb1ffcf7f6d9201514db807bc8
Reviewed-on: http://review.couchbase.org/23311
Tested-by: Sriram Melkote <3995a3476470ce2c0c01f75cee639e7d47357a62@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

t3rm1n4l/ns_server,nimishzynga/ns_server,mschoch/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,mschoch/ns_server,vzasade/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,membase/ns_server,nimishzynga/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,ceejatec/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,membase/ns_server,vzasade/ns_server,vzasade/ns_server,mschoch/ns_server,membase/ns_server,membase/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,nimishzynga/ns_server,ceejatec/ns_server,daverigby/ns_server,daverigby/ns_server