path: Also accept / as directory separator on Windows

This adds helper functions to determine the first or last directory separator
in a string and to check if a given character is a separator.

Paths starting with a separator are now also considered absolute on
Windows as these are rooted at the current drive.

Note that it's fine to use DIRECTORY_SEPARATOR when combining strings as
Windows API calls accept both forward and backward slashes as separators.

Co-authored-by: Michał Skalski <[email protected]>

References #3684.

src/libstrongswan/collections/enumerator.c

@@ -173,9 +173,9 @@ enumerator_t* enumerator_create_directory(const char *path)
 		return NULL;
 	}
 	/* append a '/' if not already done */
-	if (this->full[len-1] != '/')
+	if (!path_is_separator(this->full[len-1]))
 	{
-		this->full[len++] = '/';
+		this->full[len++] = DIRECTORY_SEPARATOR[0];
 		this->full[len] = '\0';
 	}
 	this->full_end = &this->full[len];