Reformatted code w.r.t. unifying code styles as of the official
CPPCoreGuidelines by Stroustrup and Sutter

Fixes #65.

Fixes #64.

Fixes #63.

Fixes an issue where client applications such as ssh might attempt to
close all open fds, therefore also closing the fds used by the internal
components of the library such as Mercury.

The new `LOG_SYSCALLS` Cmake flag can be used to enable verbose logging of
system calls interception.