curl: preserve UNC paths when normalizing/simplifying
curl: preserve UNC paths when normalizing/simplifying

The code to make cURL relocatable simplifies/normalizes paths, e.g.
reducing multiple forward slashes to a single one.

This is okay in general, but at the beginning of the path, a double
slash has a special meaning: it denotes a UNC path of the form
//<host>/<path>. It would therefore be a mistake to reduce those first
two slashes to a single one: otherwise it would refer to the absolute
path relative to the current directory's drive.

Let's preserve the double slashes.

Reported-in: https://github.com/git-for-windows/git/issues/3266
Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

InitialForce/MINGW-packages,fabiangreffrath/MINGW-packages,Ede123/MINGW-packages,Optiligence/MINGW-packages,Optiligence/MINGW-packages,JPeterMugaas/MINGW-packages,JPeterMugaas/MINGW-packages,fabiangreffrath/MINGW-packages,fabiangreffrath/MINGW-packages,fabiangreffrath/MINGW-packages,peterbud/MINGW-packages,JPeterMugaas/MINGW-packages,Alexpux/MINGW-packages,lhmouse/MINGW-packages,Chocobo1/MINGW-packages,oscarfv/MINGW-packages,oscarfv/MINGW-packages,jtanx/MINGW-packages,JPeterMugaas/MINGW-packages,mati865/MINGW-packages,Ede123/MINGW-packages,Alexpux/MINGW-packages,mati865/MINGW-packages,stahta01/MINGW-packages,Alexpux/MINGW-packages,mati865/MINGW-packages,Optiligence/MINGW-packages,oroppas/MINGW-packages,mati865/MINGW-packages,lhmouse/MINGW-packages,lhmouse/MINGW-packages,oroppas/MINGW-packages,lhmouse/MINGW-packages,Chocobo1/MINGW-packages,oscarfv/MINGW-packages,Alexpux/MINGW-packages,stahta01/MINGW-packages,JPeterMugaas/MINGW-packages,Alexpux/MINGW-packages,fabiangreffrath/MINGW-packages,peterbud/MINGW-packages,peterbud/MINGW-packages,oroppas/MINGW-packages,stahta01/MINGW-packages,InitialForce/MINGW-packages,jtanx/MINGW-packages,oscarfv/MINGW-packages,Chocobo1/MINGW-packages,Ede123/MINGW-packages,oscarfv/MINGW-packages,fabiangreffrath/MINGW-packages,oroppas/MINGW-packages,JPeterMugaas/MINGW-packages,oroppas/MINGW-packages,jtanx/MINGW-packages,oroppas/MINGW-packages,Optiligence/MINGW-packages,oscarfv/MINGW-packages,stahta01/MINGW-packages,jtanx/MINGW-packages,Alexpux/MINGW-packages,peterbud/MINGW-packages,InitialForce/MINGW-packages,stahta01/MINGW-packages,Ede123/MINGW-packages,lhmouse/MINGW-packages,stahta01/MINGW-packages,mati865/MINGW-packages,lhmouse/MINGW-packages