Set section indices when symbols are made

Set section indices when symbols are made

Most places in gdb that create a new symbol will apply a section
offset to the address.  It seems to me that the choice of offset here
is also an implicit choice of the section.  This is particularly true
if you examine fixup_section, which notes that it must be called
before such offsets are applied -- meaning that if any such call has
an effect, it's purely by accident.

This patch cleans up this area by tracking the section index and
applying it to a symbol when the address is set.  This is done for
nearly every case -- the remaining cases will be handled in later
patches.