Fix fsck-related denials with encrypted userdata.

Allow error reporting via the pty supplied by init.
Allow vold to invoke fsck for checking volumes.

Addresses denials such as:
avc:  denied  { ioctl } for  pid=133 comm="e2fsck" path="/dev/pts/0" dev="devpts" ino=3 scontext=u:r:fsck:s0 tcontext=u:object_r:devpts:s0 tclass=chr_file

avc: denied { execute } for pid=201 comm="vold" name="e2fsck" dev="mmcblk0p25" ino=98 scontext=u:r:vold:s0 tcontext=u:object_r:fsck_exec:s0 tclass=file

These denials show up if you have encrypted userdata.

Change-Id: Idc8e6f83a0751f17cde0ee5e4b1fbd6efe164e4c
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>