kexec: export kexec.h to user space

Add userspace definitions, guard all relevant kernel structures. While at it document stuff and remove now useless userspace hint.

It is easy to add the relevant system call to respective libc's, but it seems pointless to have to duplicate the data structures.

This is based on the kexec-tools headers, with the exception of just using int on return (succes or failure) and using size_t instead of 'unsigned long int' for the number of segments argument of kexec_load().

---
 include/linux/Kbuild  |    1 +
 include/linux/kexec.h |   75 ++++++++++++++++++++++++++++++++++---------------
 2 files changed, 54 insertions(+), 22 deletions(-)

index 3973783..d4ad37e 100644
@@ -225,6 +225,7 @@ header-y += kd.h
 header-y += kdev_t.h
 header-y += kernel.h
 header-y += kernelcapi.h
+header-y += kexec.h
 header-y += keyboard.h
 header-y += keyctl.h
 header-y += l2tp.h

1.7.10