Make read_program_headers_from_bfd return a gdb::byte_vector

Make read_program_headers_from_bfd return a gdb::byte_vector

This patch makes read_program_headers_from_bfd return a gdb::byte_vector
instead of a plain pointer.

gdb/ChangeLog:

	* solib-svr4.c (read_program_headers_from_bfd): Return
	gdb::optional<gdb::byte_vector>.
	(svr4_exec_displacement): Adjust.