cgroup: don't use fread in read_cgroup_prop()

I think this version of code is a bit more readable.
It doesn't do memcpy and doesn't allocate FILE.
Everyone knows arguments for read(), but only a few of
us know arguments for fread().

CID 73345 (#1 of 1): String not null terminated (STRING_NULL)
2. string_null_argument: Function fread does not terminate string *buf. [Note: The source code implementation of the function has been overridden by a builtin model.]

Cc: Tycho Andersen <tycho.andersen@canonical.com>
sk-inet: don't leak sockets

CID 73347 (#7-4 of 7): Resource leak (RESOURCE_LEAK)
21. leaked_handle: Handle variable sk going out of scope leaks the handle.
ipc_ns: don't leak memory

CID 73377: Resource leak (RESOURCE_LEAK)
10. leaked_storage: Variable values going out of scope leaks the storage
it points to.
proc: don't leak memory

CID 73370: Resource leak (RESOURCE_LEAK)
13. leaked_storage: Variable timer going out of scope leaks the storage it points to.