Calculate relocation base for the main object, and apply the relocation

Calculate relocation base for the main object, and apply the relocation
adjustment for all virtual addresses encoded into the ELF structures of
it. PIE binary could and should be loaded at non-zero mapbase.

For sym_zero pseudosymbol used as a return value from find_symdef()
for undefined weak symbols, st_value also should be adjusted, since
_rtld_bind corrects symbol values by relocbase.

Discussed with:	bz
Reviewed by:	kan
Tested by:	bz (i386, amd64), bsam (linux)
MFC after:	some time