blockdev-util: Remove dependency on string-util.h

blockdev-util: Remove dependency on string-util.h

Let's insist on a string literal in SYS_BLOCK_PATH_MAX() so that
we don't accidentally allocate VLAs and let's inline strempty() in
xsprintf_sys_block_path() so we don't need to include string-util.h
in blockdev-util.h

We'll remove the actual string-util.h include as part of #37344.