Introduce obstack_strndup

Introduce obstack_strndup

This introduces obstack_strndup and changes gdb to use it.

Note that obstack_strndup works like savestring, and not exactly like
xstrndup.  The difference is that obstack_strndup uses the passed-in
length, while xstrndup uses strnlen to choose the length.

gdb/ChangeLog
2019-08-06  Tom Tromey  <tom@tromey.com>

	* stabsread.c (patch_block_stabs, read_one_struct_field)
	(read_enum_type): Use obstack_strndup.
	* rust-exp.y (rust_parser::copy_name): Use obstack_strndup.
	* gdb_obstack.h (obstack_strndup): Use obstack_strndup.
	* dwarf2read.c (guess_full_die_structure_name)
	(anonymous_struct_prefix): Use obstack_strndup.
	* dbxread.c (cp_set_block_scope): Use obstack_strndup.
	* c-exp.y (yylex): Use obstack_strndup.
	* ada-exp.y (write_object_renaming, write_ambiguous_var)
	(write_var_or_type): Use obstack_strndup.