readelf: Add 'Key to Flags' to eu-readelf --section-headers output

readelf: Add 'Key to Flags' to eu-readelf --section-headers output

When printing section headers, also include a key to what each flag
is at the end of the section header output.

   * src/readelf.c (print_flag_info): New function.
   (print_shdr): Call print_flag_info.
   * tests/run-copyadd-sections.sh: Fix .extra grep by escaping \.
   * tests/run-large-elf-file.sh: Likewise.
   * tests/test-copymany-subr.sh: Likewise.
   * tests/run-readelf-z.sh: Add Key to Flags to expected output.
   * tests/run-retain.sh: Likewise.
   * tests/run-strip-remove-keep.sh: Likewise.

https://sourceware.org/bugzilla/show_bug.cgi?id=29571

Signed-off-by: Samuel Zeter <samuelzeter@gmail.com>