MySQL options for pt-mysql-summary and pt-stalk

pt-mysql-summary doesn't have MySQL options like --user, --port, etc. Users must specify these after the tool's options then a magical -- on the command line, but this is non-standard. It should be possible to add all the standard connection options (like other tools have: --user, --port, --socket, --defaults-file, --host, --database) and automatically create what's currently EXT_ARGV from any such connection option that was given (--defaults-file always being first).

The the goal is to be able to do,

pt-mysql-summary --user foo --password bar

rather than,

pt-mysql-summary -- --user foo --password bar

Right now in the code everything after -- is put in EXT_ARGV as-is so tools do things like "mysql $EXT_ARGV ...". This change would require constructing EXT_ARGV, perhaps in the parse_options.sh lib.

This also applies to pt-stalk.