Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Import jemalloc 3.2.0.

Import jemalloc a8f8d7540d66ddee7337db80c92890916e1063ca (dev branch,
prior to 3.0.0 release). This fixes several bugs related to memory
initialization.

Mangle __jemalloc_a0{malloc,calloc,free}() just like all the other
library-internal symbols in jemalloc, and adjust the tls allocation code
in libc to use the mangled names.

Import jemalloc 9ef7f5dc34ff02f50d401e41c8d9a4a928e7c2aa (dev branch,
prior to 3.0.0 release) as contrib/jemalloc, and integrate it into libc.
The code being imported by this commit diverged from
lib/libc/stdlib/malloc.c in March 2010, which means that a portion of
the jemalloc 1.0.0 ChangeLog entries are relevant, as are the entries
for all subsequent releases.