dotdot: introducing dot file path cleanup

RFC3986 details how a path part passed in as part of a URI should be
"cleaned" from dot sequences before getting used. The described
algorithm is now implemented in lib/dotdot.c with the accompanied test
case in test 1395.

Bug: http://curl.haxx.se/bug/view.cgi?id=1200
Reported-by: Alex Vinnik