Check for bad device-mapper name early

There is no need to unlock keyslot if the provided name
has wrong format. Let's check for length and '/' in name early.

Note that other commands could accept path to the device
as libdevmapper translate it to the name (status /dev/mapper/xxx).
Add early check only to activate commands.

It still can fail later because of mangled characters.

Fixes: #893

lib/libcryptsetup_macros.h

@@ -49,6 +49,7 @@
 #define DEFAULT_MEM_ALIGNMENT	4096
 
 #define DM_UUID_LEN		129
+#define DM_NAME_LEN		128
 #define DM_BY_ID_PREFIX		"dm-uuid-"
 #define DM_BY_ID_PREFIX_LEN	8
 #define DM_UUID_PREFIX		"CRYPT-"