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 <mskalski@enigma.com.pl>

References #3684.

src/libimcv/plugins/imv_attestation/attest_db.c

@@ -337,7 +337,7 @@ METHOD(attest_db_t, set_directory, bool,
 
 	/* remove trailing '/' or '\' character if not root directory */
 	len = strlen(dir);
-	if (len > 1 && dir[len-1] == DIRECTORY_SEPARATOR[0])
+	if (len > 1 && path_is_separator(dir[len-1]))
 	{
 		dir[len-1] = '\0';
 	}