Define behavior of globstar (**)

[joachimmetz]

Define behavior of globstar (**)

Bash supports:

• stand-alone **, behaves like */**
• a prefix d**, behaves like d*/**
• a suffix **c, behaves like *c/**
• a prefix and suffix d**c, behaves like d*c/**

From bash manual

*      Matches any string, including the null string.  When the globstar shell option is enabled, and * is used in a pathname expansion context, two adjacent *s used as a single pattern will match all files and zero or
       more directories and subdirectories.  If followed by a /, two adjacent *s will match only directories and subdirectories.

[joachimmetz]

Todo: look at behavior of GRR

https://github.com/google/grr/blob/master/grr/client/grr_response_client/client_actions/file_finder_utils/globbing_test.py

• GRR defaults ** to **3 https://github.com/google/grr/blob/master/grr/client/grr_response_client/client_actions/file_finder_utils/globbing.py#L65
• GRR does not appear to support a globstar with a prefix and/or suffix https://github.com/google/grr/blob/master/grr/client/grr_response_client/client_actions/file_finder_utils/globbing_test.py#L523
• GRR does appear support a path with multiple globstars https://github.com/google/grr/blob/master/grr/client/grr_response_client/client_actions/file_finder_utils/globbing_test.py#L560

Todo: get clarification on ** does not match the current directory.

Todo: what about Registry paths

[joachimmetz]

For now only support paths with a single globstar formatted as: **, **2 upto **10

**1 makes no sense to support

Changes to have validator enforce this #344

Todo: consider to support path with multiple globstars
Todo: define default recursion depth, or discourage usage of globstar without recursion depth