Device with non mcblk partition table (Galaxy S6)

Hello,

First: Thanks for the script!

I have a Galazy S6 in my possession and they named the partition list as 'sda#' not 'mcblk0p#.
Listing partitions in adb shell returns:
```bash
~ # ls -l /dev/block/platform/15570000.ufs/by-name/

lrwxrwxrwx 1 root root 15 Nov 7 11:03 BOOT -> /dev/block/sda5
lrwxrwxrwx 1 root root 15 Nov 7 11:03 BOTA0 -> /dev/block/sda1
lrwxrwxrwx 1 root root 15 Nov 7 11:03 BOTA1 -> /dev/block/sda2
lrwxrwxrwx 1 root root 16 Nov 7 11:03 CACHE -> /dev/block/sda16
lrwxrwxrwx 1 root root 16 Nov 7 11:03 DNT -> /dev/block/sda10
lrwxrwxrwx 1 root root 15 Nov 7 11:03 EFS -> /dev/block/sda3
lrwxrwxrwx 1 root root 16 Nov 7 11:03 HIDDEN -> /dev/block/sda17
lrwxrwxrwx 1 root root 15 Nov 7 11:03 OTA -> /dev/block/sda7
lrwxrwxrwx 1 root root 15 Nov 7 11:03 PARAM -> /dev/block/sda4
lrwxrwxrwx 1 root root 16 Nov 7 11:03 PERSDATA -> /dev/block/sda13
lrwxrwxrwx 1 root root 16 Nov 7 11:03 PERSISTENT -> /dev/block/sda11
lrwxrwxrwx 1 root root 15 Nov 7 11:03 RADIO -> /dev/block/sda8
lrwxrwxrwx 1 root root 15 Nov 7 11:03 RECOVERY -> /dev/block/sda6
lrwxrwxrwx 1 root root 16 Nov 7 11:03 SBFS -> /dev/block/sda14
lrwxrwxrwx 1 root root 16 Nov 7 11:03 STEADY -> /dev/block/sda12
lrwxrwxrwx 1 root root 16 Nov 7 11:03 SYSTEM -> /dev/block/sda15
lrwxrwxrwx 1 root root 15 Nov 7 11:03 TOMBSTONES -> /dev/block/sda9
lrwxrwxrwx 1 root root 16 Nov 7 11:03 USERDATA -> /dev/block/sda18
```

```bash
~ # cat /proc/partitions
major minor #blocks name

8 0 31240192 sda
8 1 4096 sda1
8 2 4096 sda2
8 3 20480 sda3
8 4 8192 sda4
8 5 28672 sda5
8 6 34816 sda6
8 7 8192 sda7
8 8 43008 sda8
8 9 1024 sda9
8 10 1024 sda10
8 11 768 sda11
8 12 256 sda12
8 13 9216 sda13
8 14 15360 sda14
8 15 3788800 sda15
259 0 204800 sda16
259 1 10240 sda17
259 2 27045888 sda18
8 32 4096 sdc
8 16 4096 sdb
```

So a few adjustments to your script were needed in `build_partmap`
```python
mmcblks = adb.check_output(('shell','cd /sys/block; ls -d mmcblk*')).split()
```
to
```python
mmcblks = adb.check_output(('shell','cd /sys/block; ls -d sda*')).split()
```
and removing the 'p' in the calls to specific partitions : '%s**p**%d' % (mmcblk, partn) to '%s%d' % (mmcblk, partn)

running `tetherback -0` works fine afterwards:
```bash
$ python3 -m tetherback -0
__main__.py v0.9.1
Found ADB version 1.0.36
Using default transfer method: adb exec-out pipe (--exec-out)
Device reports kernel 3.10.101-arter97-13.1-04960-ge491d042
Device reports TWRP version 3.0.2-0
Reading partition map for sda (18 partitions)...
partition map: 100%

Partition map:

BLOCK DEVICE PARTITION NAME SIZE (KiB) MOUNT POINT FSTYPE
-------------- ---------------- ------------ ------------- --------
sda1 bota0 4096
sda2 bota1 4096
sda3 efs 20480
sda4 param 8192
sda5 boot 28672
sda6 recovery 34816
sda7 ota 8192
sda8 radio 43008
sda9 tombstones 1024
sda10 dnt 1024
sda11 persistent 768
sda12 steady 256
sda13 persdata 9216
sda14 sbfs 15360
sda15 system 3788800 /system ext4
sda16 cache 204800 /cache ext4
sda17 hidden 10240
sda18 userdata 27045888 /data ext4
Total: 31228928

Backup plan:

PARTITION NAME FILENAME FORMAT
---------------- --------------- ------------------------------
boot boot.emmc.win gzipped raw image
system system.ext4.win tar -cz -p
userdata data.ext4.win tar -cz -p --exclude="*-cache"
```

Also I don't know if it is intended but in my case `really_umount` in `backup_partition` is called for every partition whether it is mounted or not resulting in:
`subprocess.CalledProcessError: Command '('adb', '-d', 'shell', 'umount /dev/block/sda5 2>/dev/null && echo ok')' returned non-zero exit status 1`
As (I) expected, running the command in adb shell returns nothing if partition not mounted and 'ok' otherwise.

Dirty trick is to comment the `really_umount` part.

Also got same problem as issue #28 when doing a TWRP backup, no problem with nandroid.

Don't know if you plan on updating your script for this particular case but I thought others could be interested.