The new API is defined by GNU and implemented in other LibCs, such as
Musl.
This also modifies API of obstack_blank_fast and obstack_1grow_fast.
These are defined as returning void and thus return value here is
incompatibility with other implementations.
obstack_printf and obstack_vprintf should terminate the C string with
null byte but lib_vsprintf doesn't do it. It must be done on top of
that unless we get unterminated string.
This is fix to be consistent with GlibC behavior.
This also includes minor tweak to use obstack_1grow directly instead of
calling obstack_puts.
