Create target sections for user-added symbol files.

Create target sections for user-added symbol files.

Add the sections of the symbol files that are provided via
'add-symbol-file' to the set of current target sections.
User-added sections are removed upon notification of free_objfile
when their corresponding object file is deleted.

2013-10-29  Nicolas Blanc  <nicolas.blanc@intel.com>

	* exec.h (add_target_sections_of_objfile): New declaration.
	* exec.c (add_target_sections_of_objfile): New function.
	* symfile.c (add_symbol_file_command): Update current target sections.
	(symfile_free_objfile): New function.
	(_initialize_symfile): Register observer for free_objfile events.

Signed-off-by: Nicolas Blanc <nicolas.blanc@intel.com>