Fix PIT Interrupt Not Context Switching [irq]

Set the counter to 0 when context is to be switched in the PIT interrupt,
and change the condition for context switching from % 10 to >= 10.

PIT interrupt should context switch or else all of redox crashes.
This will fix programs like the Snake game crashing all of Redox.
A global AtomicUSize counter was added, and a line to switch contexts
on every 10 PIT interrupts in irq.rs.

Add fcntl to schemes
Fix debug: hang

A faster implementation of the memcpy family

Instead of multiplying everything by 8[/4], now addition is used. That
way code is prettier.

The default implementation of the memcpy, memmove, memset and memcmp
functions in the kernel file `extern.rs` uses a naive implementation
by copying, assigning or comparing bytes ony by one. This can be slow.
This commit proposes a reimplementation of those functions by copying,
assigning or comparing in group of 8 bytes by using the u64 type and
its respective pointers instead of u8. Alternative version for 32-bit
architectures are also supplied for future compatibility with x86.
Both version first copy whatever they can with wide word types. The
tail, i.e. the final few bytes that do not fit in a dword or qword
are then copied byte by byte.

Here is a comparison of copying 64kiB (65536 bytes) on stack:

x86_64-unknown-linux-gnu: (64-bit)
       | naive (ns) | fast (ns) | speedup (x)
-------|------------|-----------|------------
memcpy |   204430   |   32994   |   ~6.20
memmove|   202540   |   33186   |   ~6.10
memset |   163391   |   23884   |   ~6.84
memcmp |   205663   |   34385   |   ~5.98

i686-unknown-linux-gnu: (32-bit)
       | naive (ns) | fast (ns) | speedup (x)
-------|------------|-----------|------------
memcpy |   206297   |   66858   |   ~3.09
memmove|   204576   |   70326   |   ~2.91
memset |   165599   |   50227   |   ~3.30
memcmp |   204262   |   70572   |   ~2.89

Copying on the heap behaves simmilarly.

All tests performed on Intel i5 6600K (4x4.2GHz),
ArchLinux Kernel 4.8.12-3 x86_64.

Fix the listing of `initfs:` directories

There was a bug at the `initfs` generation which made the listing of the contents of the `initfs:`subdirectories impossible from the command line.

The subdirectory listing data had the full paths of the files (like `bin/ahcid\nbin/bgad\n...`) when it should just only be the names of the files (`ahcid\nbgad\n...`)

Cleanup utf8 path error handling

Refactor

Fix spelling & grammar in README.md

Fix issue with serial and cascade interrupts not being ackd