Fix: conversion from KB to bytes overflow on arm32

Fix: conversion from KB to bytes overflow on arm32

Observed issue
==============

On enable channel the memory available check fails on arm32 when
available memory, in bytes, is larger than 2^32.

Cause
=====

`read_proc_meminfo_field` converts the read value (in KB) to bytes and
stores it into a size_t variable.

On the system running the reproducer the value of the `value_kb` variable
is 4839692, yielding an overflow when multiplied with 1024 since
`size_t` is 32 bit long. `size_t` can be larger in certain situation
(i.e LARGEFILE) but this is irrelevant to the problem at hand.

Solution
========

Convert all the checks to use uint64_t.

Known drawbacks
=========

None.

References
==========

The multiplication overflow check scheme is borrowed from
`src/common/time.c`

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I76fe7e57da149c1b4c230a411e0346ba6f9ef7b1