remote-vx.c, remote.c, target.c, target.h: Remove add_syms vector
from target_ops. It's the same on all targets. Add two section
pointers to target_ops.
target.c: Add target_preopen and target_command.
Remove target_info.
(add_target): Call add_cmd and mess with targetlist->doc.
core.c, exec.c, remote-eb.c, remote-nindy.c, remote-vx.c,
remote-vx.68.c, inftarg.c, remote.c: Add doc field to target struct.
Call target_preopen from open routine.
