rgw/rgw-gap-list: refactor / add more err checks

rgw/rgw-gap-list: refactor / add more err checks

- rename function `log_out` to `log` to prevent mental confusion with
  'logging out' of the shell, vs sending a log message.

- Centralize error checking to provide more consistent output and ease
  of error check implementation

- Add check that supplied pool name(s) exist.

- Relocate the awk script creation to just before it's needed.

- Provide usage help for specifying multiple pools on the command line

- Add hostname to log output lines.

- add checks to ensure intermediate files are not empty before
  attempting comparison.

- switch to getopt to allow  more option flexibility, removing
  dependency on positional arguments which could cause parsing / user
  confusion.
  This is a **BREAKING change** due to change in command line syntax.

- add option for temp directory specification on the command line.

- set -m option to be presence based, vs needing 0|1 param

- fix typo in usage

Signed-off-by: Michael J. Kidd <linuxkidd@gmail.com>