replace magic number for in-band meta size with a macro

add glue hook for disabling aligned_alloc

refactor conditional skipping of memset in mmap-serviced calloc

move calloc to its own file

only skip use of fully-free groups if they're freeable

fix possible stale mask in deciding whether to activate more slots

mark index byte at zero offset when aligning at nonzero offset

declare alloc_meta in meta.h, make it hidden

ignore mprotect failure on ENOSYS (nommu)

make alloc_meta externally visible

use feature test macro to get madvise in free.c

avoid reuse of slot offsets after group is freed/recreated

use already-touched pages in a group before dirtying new ones

factor group activation to a small shared inline function

fix check for impossible condition in try_avail to be an assertion