Clarify error for slow `cpu_sup` port init

I noticed that running an R16B03-1 node on an overloaded host produced
log entries like the following ones:

```
2014-08-22 21:52:31 =ERROR REPORT====
Error in process <0.24112.3> on node '...@127.0.0.1' with exit value: {{case_clause,{data,4711}},[{cpu_sup,get_uint32_measurement,2,[{file,"cpu_sup.erl"},{line,227}]},{cpu_sup,measurement_server_loop,1,[{file,"cpu_sup.erl"},{line,585}]}]}
```

```
===== ALIVE Fri Aug 22 21:50:14 CEST 2014
[os_mon] cpu supervisor port (cpu_sup): Erlang has closed
[os_mon] cpu supervisor port (cpu_sup): Erlang has closed
[os_mon] cpu supervisor port (cpu_sup): Erlang has closed
[os_mon] cpu supervisor port (cpu_sup): Erlang has closed

===== ALIVE Fri Aug 22 22:07:46 CEST 2014
```

I performed a code inspection on the `cpu_sup` module and I concluded
that the `case_clause` error shows a small issue in the `cpu_sup`
module, that happens when the port used in `cpu_sup` is slow to start
- as it may happen on an overloaded node.

The `cpu_sup` `gen_server` process keeps in its state the pid of an
unlinked process (called "measurement server" - see `cpu_sup:init/1`),
in order to do dirty stuff (e.g. reading the filesystem, running OS
commands) and start_link-ing & managing the connected process to a
port (called "port server" - see `measurement_server_init/0`). So the
process organization looks like this:

```
cpu_sup - measurement server - port server - port
```

When the measurement server start_links the port server (see
`port_server_start/0`) it sends a `{self(), ?ping}` message to it and
expects an answer within 6s, otherwise it returns `{error, timeout}`
rather than the pid. This has two issues:

* The measurement server keeps `{error, ...}` in its state as if it
  were a pid - that makes no sense;

* A late `{Pid, {data,4711}}` response may arrive in the mailbox of
  the measurement server, that will believe it to be a request to be
  processed, causing a `case_clause` error.

This commit teaches the measurement server to check the success of the
initialization of the port server by matching on the return value of
`port_server_start/0` (renamed to `port_server_start_link/0` for the
sake of clarity) in order to fail earlier and with an error clearer
than `{case_clause,{data,4711}}`. In such case I expect the
measurement server to be restarted by the `cpu_sup` `gen_server` (see
`handle_call/3`) - as before.

BTW It is not clear to me when the `handle_info({'EXIT', _Port,
Reason}, State)` may be called (the `cpu_sup` `gen_server` does not
link to the measurement server) but I am leaving it.