Make read_program_header return a gdb::byte_vector

Make read_program_header return a gdb::byte_vector

While reading a recent patch, I found this spot where a gdb::byte_vector
could be used instead of an allocated buffer returned as a plain
pointer.

gdb/ChangeLog:

	* solib-svr4.c (read_program_header): Return
	gdb::optional<gdb::byte_vector>, remove p_sect_size param.
	(find_program_interpreter): Return
	gdb::optional<gdb::byte_vector>.
	(scan_dyntag_auxv): Adjust.
	(enable_break): Adjust.
	(svr4_exec_displacement): Adjust.