Fix comments in _dl_dst_count and _dl_dst_substitute.

The comments in _dl_dst_count is adjusted to match what the code does
which is count DSTs from the start of the string. With the removal of
DL_DST_COUNT we no longer accept an input that starts at the first $.

In _dl_dst_substitute we adjust the comment to indicate that both
conditions must be true for the SUID/SGID $ORIGIN exception.