Minor ABLOCKS_BUSY cleanups in alloc.c

Minor ABLOCKS_BUSY cleanups in alloc.c

* src/alloc.c (ABLOCKS_BUSY): Rename arg to avoid potential clash
with member ‘abase’ in definiens.
(lisp_align_malloc, lisp_align_free): Use bool for boolean.
Avoid compiler warning with fewer casts.
(lisp_align_free): Check busy-field values; this can help the
compiler a bit when optimizing, too.