elflint: Use shnum and shstrndx instead of ehdr field directly.

elflint: Use shnum and shstrndx instead of ehdr field directly.

We already got the right shnum and shstrndx. But were still using
e_shnum in one check for ELFCLASS64 (it was correct for ELFCLASS32).
And when getting section names in check_symtab we still used
e_shstrndx in two places.

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