New features:

• Auto-detect whether running inside Valgrind, thus removing the need to manually specify MALLOC_CONF=valgrind:true.
• Add the "arenas.extend" mallctl, which allows applications to create manually managed arenas.
• Add the ALLOCM_ARENA() flag for {,r,d}allocm().
• Add the "opt.dss", "arena.<i>.dss", and "stats.arenas.<i>.dss" mallctls, which provide control over dss/mmap precedence.
• Add the "arena.<i>.purge" mallctl, which obsoletes "arenas.purge".
• Define LG_QUANTUM for hppa.

Incompatible changes:

• Disable tcache by default if running inside Valgrind, in order to avoid making unallocated objects appear reachable to Valgrind.
• Drop const from malloc_usable_size() argument on Linux.

Bug fixes:

• Fix heap profiling crash if sampled object is freed via realloc(p, 0).
• Remove const from __*_hook variable declarations, so that glibc can modify them during process forking.
• Fix mlockall(2)/madvise(2) interaction.
• Fix fork(2)-related deadlocks.
• Fix error return value for "thread.tcache.enabled" mallctl.