Added in kernel 5.8

This was previously causing an errno of 115 during tests. While an errno of 0 is not guaranteed on success, all errno's within GKFS should be kept internal.

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

Restructuring code w.r.t. configurations and definitions:
- #defines have been mostly removed from configurations
- a dedicated config file has been added for configurations with constexpr
- past configure file is now only a cmake wrapper
- wrapping global functions into namespaces

Removed all adafs and ifs occurrences. Now called gkfs

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.

lseek was triggering a number overflow by converting long to integer on
its return value

Added specific lseek test