gdb: allocate symfile_segment_data with new

gdb: allocate symfile_segment_data with new

- Allocate this structure with new instead of XNEW, use a unique pointer
  to manage its lifetime.
- Change a few functions to return a unique   pointer instead of a
  plain pointer.
- Change free_symfile_segment_data to be symfile_segment_data's
  destructor.

gdb/ChangeLog:

	* symfile.h (struct symfile_segment_data): Initialize fields.
	<~symfile_segment_data>: Add.
	(symfile_segment_data_up): New.
	(struct sym_fns) <sym_segments>: Return a
	symfile_segment_data_up.
	(default_symfile_segments): Return a symfile_segment_data_up.
	(free_symfile_segment_data): Remove.
	(get_symfile_segment_data): Return a symfile_segment_data_up.
	* symfile.c (default_symfile_segments): Likewise.
	(get_symfile_segment_data): Likewise.
	(free_symfile_segment_data): Remove.
	(symfile_find_segment_sections): Update.
	* elfread.c (elf_symfile_segments): Return a
	symfile_segment_data_up.
	* remote.c (remote_target::get_offsets): Update.
	* solib-target.c (solib_target_relocate_section_addresses):
	Update.
	* symfile-debug.c (debug_sym_segments): Return a
	symfile_segment_data_up.