RMCALL: allow real mode interface to be relocatable in whole first MB of memory

initially it was possible to place the data only in the first 64kB
It is done by changing base of 16-bit protected mode and data structures
(offset to jump to and base of passed registers)
RMCALL: do not use unnecessary base in indirect addressing
bsp.h: remove not lucid asm guards->move GDT,IDT tbl sizes to its own header

sizes need to be accessible from asm file ldsegs.S
i386: physical_to_real_mode_ptr usage example
RMCALL: use C function for copying instead of ASM
RMCALL: structures refactorization + comments
VBE_RM: remove unused variables
GDT: documentation upgrade
GDT: only static allocation of GDT descriptors, removed unnecesary functions
GDT: descriptor is valid only after all other values were filled
VBE_RM: error if call to real mode interrupt fails

error is produced only on first attempt to call real mode interrupt (VBE
function) if it passed once it should pass other times too

more obvious return values from VBE functions

ret_vbe refactored to vbe_ret_val