Use a std::vector in target_section_table

This changes target_section_table to wrap a std::vector.  This
simplifies some code, and also enables the simplifications coming in
the subsequent patches.

Note that for solib, I chose to have it use a pointer to a
target_section_table.  This is more convoluted than would be ideal,
but I didn't want to convert solib to new/delete as a prerequisite for
this series.

gdb/ChangeLog
2020-10-12  Tom Tromey  <tom@tromey.com>

	* target.c (target_section_by_addr, memory_xfer_partial_1):
	Update.
	* target-section.h (struct target_section_table): Use
	std::vector.
	* symfile.h (build_section_addr_info_from_section_table): Take a
	target_section_table.
	* symfile.c (build_section_addr_info_from_section_table): Take a
	target_section_table.
	* solist.h (struct so_list) <sections>: Change type.
	<sections_end>: Remove.
	* solib.c (solib_map_sections, clear_so, solib_read_symbols)
	(solib_contains_address_p): Update.
	* solib-svr4.c (scan_dyntag): Update.
	* solib-dsbt.c (scan_dyntag): Update.
	* remote.c (remote_target::remote_xfer_live_readonly_partial):
	Update.
	* record-full.c (record_full_core_start, record_full_core_end):
	Remove.
	(record_full_core_sections): New global.
	(record_full_core_open_1, record_full_core_target::xfer_partial):
	Update.
	* exec.h (build_section_table, section_table_xfer_memory_partial)
	(add_target_sections): Take a target_section_table.
	* exec.c (exec_file_attach, clear_section_table): Update.
	(resize_section_table): Remove.
	(build_section_table, add_target_sections): Take a
	target_section_table.
	(add_target_sections_of_objfile, remove_target_sections)
	(exec_on_vfork): Update.
	(section_table_available_memory): Take a target_section_table.
	(section_table_read_available_memory): Update.
	(section_table_xfer_memory_partial): Take a target_section_table.
	(print_section_info, set_section_command)
	(exec_set_section_address, exec_target::has_memory): Update.
	* corelow.c (class core_target) <m_core_section_table,
	m_core_file_mappings>: Remove braces.
	<~core_target>: Remove.
	(core_target::core_target): Update.
	(core_target::~core_target): Remove.
	(core_target::build_file_mappings)
	(core_target::xfer_memory_via_mappings)
	(core_target::xfer_partial, core_target::info_proc_mappings):
	Update.
	* bfd-target.c (target_bfd::xfer_partial): Update.
	(target_bfd::target_bfd): Update.
	(target_bfd::~target_bfd): Remove.