malloc: Enable merging of remainders in memalign (bug 30723)

Previously, calling _int_free from _int_memalign could put remainders
into the tcache or into fastbins, where they are invisible to the
low-level allocator.  This results in missed merge opportunities
because once these freed chunks become available to the low-level
allocator, further memalign allocations (even of the same size are)
likely obstructing merges.

Furthermore, during forwards merging in _int_memalign, do not
completely give up when the remainder is too small to serve as a
chunk on its own.  We can still give it back if it can be merged
with the following unused chunk.  This makes it more likely that
memalign calls in a loop achieve a compact memory layout,
independently of initial heap layout.

Drop some useless (unsigned long) casts along the way, and tweak
the style to more closely match GNU on changed lines.

Reviewed-by: DJ Delorie <dj@redhat.com>
(cherry picked from commit 542b1105)