Adds dr_get_app_args() to enable a client to retrieve the locations where command-line arguments of the app reside. C strings of the arguments are retrieved via a new function, namely dr_app_arg_as_cstring(). Also, dr_num_app_args() enables the client to get the number of arguments.
The PR adds support only for Unix and early injection for now.
The gathering of error codes of failed DR API routines is also added with dr_get_error_code().
Issue: #2662.