Make CLK_NULL_RESERVE_ID invalid reserve id.
Rename PIPE_RESERVE_ID_VALID_BIT to avoid user name space pollution.
Current implementation reserve_id_t type assumes that it's a pointer
type whose most significant bit is set to one and the rest of the bits
keep the id value.
This patch increase reserve id size by one bit on 32-bit platforms and
by 33 bits on 64-bit platforms.