strip,unstrip: Use and set shdrstrndx consistently.

strip,unstrip: Use and set shdrstrndx consistently.

In various places in strip we used e_shstrndx instead of shdrstrndx and we
didn't setup the shdrstrndx for the debug file. In unstrip we forgot to copy
the shdrstrndx in case the -o output option was used.

Added a new testcase that adds many sections to a testfile and runs strip, elflint,
unstrip and elfcmp.

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