Changes the port parsing from `/proc/net/*` files records from parsing them as 64-bit integers to parse them as 16-bit unsigned integers.

While this is mostly a cosmetic change, it will also make so that the code fails faster in case the entry is malformed (for whatever reason).

Note that the returned value is still casted to uint32 when an  `Addr` object is created.
It seems to me that the `Addr.port` field should be changed to `uint16` but maybe some other APIs/systems wants it to be `uint32` and also changing it there may require changes in users code if they update. This being said I am not changing that field's type.

`connectionsList, err := net.ConnectionsMax("tcp4", 1000)`
when you run net.ConnectionsMax,you will find some proc is not equal with the `netstat -lptn`

Signed-off-by: NJoe Stringer <joe@cilium.io>

Found using https://go-critic.github.io/overview#emptyFallthrough-ref

This fixes #433

The /proc/net files are not guaranteed to be consistent, they are only
consitent on the row level. This is probably one of the reasons why
consequent read calls might return duplicate entries - the kernel is
changing the file as it is being read. In certain situations this might
lead to loop like situations - the same net entry is being returned when
reading the file as new connections are added to the kernel tcp table, i.e
there can be a lot of duplications.

This commit is trying to reduce the duplications, by fetching the contents
of the net files with a single read syscall.

Instead of encoding a JSON string of each connection (non-trivial at high
connection volumes) we can use the connTmp struct for map look-ups if we
eliminate the unused `uids` field.

Also switches to using the empty struct instead of bool for zero memory
overhead.

The goal is to improve performance of connection fetching connections across
all processes when some processes can have several hundred or thousands of file
descriptors. Right now when you have many thousands of fds the process spends
lots of time inside the syscalls from Readdir and Readlink.

The public API works as before with two new functions:

- `ConnectionsMax`
- `ConnectionsPidMax`

Each function takes an additional int argument that sets the max number of fds
read per process.

fixes #284