Fixes in comments and debug messages:

  existance -> existence
  unecessary -> unnecessary
  occured -> occurred
  decriptor -> descriptor
  neccessary -> necessary
  addres, adress -> address
  initilize -> initialize
  sucessfully -> successfully
  paramter -> parameter
  acess -> access
  upto -> up to
  likelyhood ->likelihood
  thru -> through
  substracting -> subtracting
  lenght -> length
  isnt -> isn't
  interupt -> interrupt
  publically -> publicly (this one was not wrong, but unusual)
  recieve -> receive
  accessable -> accessible
  seperately -> separately
  pacet -> packet
  controled -> controlled
  dectect -> detect
  indicies -> indices
  extremly -> extremely
  boundry -> boundary
  usefull -> useful
  unuseable -> unusable
  auxilliary -> auxiliary
  embeded -> embedded
  enviroment -> environment
  sturcture -> structure
  complier -> compiler
  constructes -> constructs
  supress -> suppress
  intruduced -> introduced
  compatability -> compatibility
  verfication -> verification
  ths -> the
  reponse -> response

Fixes in local variable names:

  retreive -> retrieve

Most of these fixes were made using codespell.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.