The helpers.h header specifies the BPF program license as 'GPL'.
However, other GPL-compatible licenses are possible (e.g., Dual BSD/GPL)
or even proprietary licenses (e.g., cachetop can run with a proprietary
license).

With this commit, the user can specify a BPF_LICENSE macro in the source
code:

  #define BPF_LICENSE Custom license

Note it supports multiple words and the absence of quotes. If the BPF
doesn't have a GPL-compatible license and it uses any GPL-only helpers,
the kernel will reject it with:

  cannot call GPL only function from proprietary program

If no license is specified, it will fall back to GPL (the current
behavior before this commit) so that the BCC tools and examples remain
usable.

Updated the documentation with BPF_LICENSE description, licensing error
description, and licenses for each helper.
Signed-off-by: Oriol Arcas <oriol@starflownetworks.com>