Concatenate a relative path to an absolute path and remove any
"//", "/.." or "/." to produce a straightforward
absolute path as result.
combine_path_nt() concatenates according to NT-filesystem conventions,
while combine_path_unix() concatenates according to UNIX-style.
combine_path() is equvivalent to combine_path_unix() on UNIX-like
operating systems, and equvivalent to combine_path_nt() on NT-like
operating systems.