cifs: cleanup misc.c

misc.c was getting a little large, move two of the UNC parsing relating
functions to a new C file unc.c which makes the coding of the
upcoming witness protocol patch series a little cleaner as well.

Suggested-by: Rafal Szczesniak <rafal@elbingbrewery.org>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>