misc: Add -h/--help options and end marker to demangler_options

[kangtegong]

This commit adds support for the -h and --help options to display usage of misc/demangler like follows.

  $ misc/demangler -h
  demangler v0.11-683-g804a

   OPTION:
        --simple           Use internal simple demangler (default)
        --full             Use libstdc++ demangler
        --no               Do not use demangler
    -v, --verbose          Be verbose
    -h, --help             Display this help and exit

An end marker has also been added to the demangler_options array for proper termination and prevent potential undefined behavior.

Fixed: #1939

[honggyukim]

Thanks @kangtegong!