sgdisk: devpts and reload partition tables.

Add rules to let sgdisk read/write to pts when forked from vold.

avc: denied { read write } for path="/dev/pts/14" dev="devpts" ino=17 scontext=u:r:sgdisk:s0 tcontext=u:object_r:devpts:s0 tclass=chr_file permissive=0

Also add rule to let it kick kernel to reload partition tables after
we finish editing them.  Without this capability, it leaves this
message and violation:

Warning: The kernel is still using the old partition table.
The new table will be used at the next reboot.
GPT data structures destroyed! You may now partition the disk using fdisk or
other utilities.

avc: denied { sys_admin } for capability=21 scontext=u:r:sgdisk:s0 tcontext=u:r:sgdisk:s0 tclass=capability permissive=0

Change-Id: If26a40f9fd3b1ab2c50156ae8bdb128676521b57