Style guide

Documented under Documentation/CodingStyle.

Style used for the Linux Kernel.

Tabs instead of spaces. Configure editors to view tabs as 8 spaces. In vim you could source:
```
if expand('%:e') =~ '\(c\|cpp\|f\)'
    set noexpandtab
    set tabstop=8
    set shiftwidth=8
endif
```
The 8 space rule is needed when we want to make ASCII tables and align each column at a multiple of the tab width so that it is easier to write the table.

For example, if a tab has 8 spaces then only one tab is need for:
```
  123456     c2
  c1          c2
  c1          c2
```
but two tabs would be needed for:
```
  123456789     c2
  c1               c2
  c1               c2
```
if the tab was sees as say, 4 spaces, the first example would look ugly:
```
  123456     c2
  c1     c2
  c1     c2
```

Double underscores

Functions that start with two underscores are low level functions. This means that:

there is probably a more convenient and usually more correct function available.
it is more likely to get deprecated some day.

The message is then clear: avoid using those unless you know exactly what you are doing and you really need to do it.