More gdb/skip.c C++ification
    
    - Make skiplist_entry a class with private data members.
    - Move all construction logic to the ctor.
    - Make skip_file_p etc be methods of skiplist_entry.
    - Use std::list for the skip entries chain.  Make the list own its
      elements.
    - Get rid of the ALL_SKIPLIST_ENTRIES/ALL_SKIPLIST_ENTRIES_SAFE
      macros, use range-for / iterators instead.
    - function_name_is_marked_for_skip 'function_sal' argument must be
      non-NULL, so make it a reference instead.
    
    All skiplist_entry invariants are now controlled by skiplist_entry
    methods/internals.  Some gdb_asserts disappear for being redundant.
    
    gdb/ChangeLog:
    2017-08-11  Pedro Alves  <palves@redhat.com>
    
    	* infrun.c (process_event_stop_test): Adjust
    	function_name_is_marked_for_skip call.
    	* skip.c: Include <list>.
    	(skiplist_entry): Make it a class with private fields, and
    	getters/setters.
    	(skiplist_entry_chain): Delete.
    	(skiplist_entries): New.
    	(skiplist_entry_count): Delete.
    	(highest_skiplist_entry_num): New.
    	(ALL_SKIPLIST_ENTRIES, ALL_SKIPLIST_ENTRIES_SAFE): Delete.
    	(add_skiplist_entry): Delete.
    	(skiplist_entry::skiplist_entry): New.
    	(skiplist_entry::add_entry): New.
    	(skip_file_command, skip_function): Adjust.
    	(compile_skip_regexp): Delete.
    	(skip_command): Don't compile regexp here.  Adjust to use
    	skiplist_entry::add_entry.
    	(skip_info): Adjust to use range-for and getters.
    	(skip_enable_command, skip_disable_command): Adjust to use
    	range-for and setters.
    	(skip_delete_command): Adjust to use std::list.
    	(add_skiplist_entry): Delete.
    	(skip_file_p): Delete, refactored as ...
    	(skiplist_entry::do_skip_file_p): ... this new method.
    	(skip_gfile_p): Delete, refactored as ...
    	(skiplist_entry::do_gskip_file_p): ... this new method.
    	(skip_function_p, skip_rfunction_p): Delete, refactored as ...
    	(skiplist_entry::skip_function_p): ... this new method.
    	(function_name_is_marked_for_skip): Now returns bool, and takes
    	the function sal by const reference.  Adjust to use range-for and
    	skiplist_entry methods.
    	(_initialize_step_skip): Remove references to
    	skiplist_entry_chain, skiplist_entry_count.
    	* skip.h (function_name_is_marked_for_skip): Now returns bool, and
    	takes the function sal by const reference.