Fully implemented reading the RTC century counter, and laid out initial infrastructure for ACPI information to be used across the kernel, in the x86_64 architecture.

- Implemented a global variable, ACPI_TABLE, behind a mutex, which contains the ACPI information pertinent to the rest of the kernel, currently solely containing a pointer to the FADT.
- Split device initialization into two categories - "core" devices, such as the PIC and local APIC, necessary for initializing the rest of the kernel, and "non-core" devices such as serial and RTC, which are to be initialized last.
- Checked for the presence of the century register, and consequentially read from, in the RTC code, now factored into the date calculation. The location of the register is pulled from the "century" field in the FADT.
- Modified page unmapping in the ACPI code, such that any tables to be stored globally (currently only the FADT) are not unmapped after reading, such that they can be stored in globally accessible pointers without causing page faults.

Prevent cpuid get_extended_function_info panic

A kernel panic occurs on some CPUs (notably qemu-system-x86_64) when
calling rust-cpuid's get_extended_function_info.  This is fixed upstream
in commit c3ebfc553cdff98d19d29777fd85c4f9182bfb66 but has yet to make
it crates.io.

The panic can by triggered by running "ls sys:" from ion and causes
redox to become unresponsive.

Remove hardcoded initfs folder

initfs folder should be specified in INITFS_FOLDER environment variable
As a result kernel module can be compiled on it's own.

remove unused #[macro_use]

Allow for more complicated directory paths

Add assertions to verify that grants are unmapped
Fix grant unmapping in exec and exit, thus fixing some crashes without network cards