readelf: Add --dyn-sym option.

readelf: Add --dyn-sym option.

It is already possible to select the symbol table to print by name,
using --symbols=SECTION. This allows printing the dynamic symbol table
with --symbols=.dynsym. binutils readelf allows printing just the
dynamic symbol table by type using --dyn-sym. Add the same option
and document it. Also add a testcase to show --symbols=.dynsym and
--dyn-sym produce the same output.

Signed-off-by: Mark Wielaard <mark@klomp.org>