Remove --enable-ivsalloc.

Continue to use ivsalloc() when --enable-debug is specified (and add
assertions to guard against 0 size), but stop providing a documented
explicit semantics-changing band-aid to dodge undefined behavior in
sallocx() and malloc_usable_size().  ivsalloc() remains compiled in,
unlike when #211 restored --enable-ivsalloc, and if
JEMALLOC_FORCE_IVSALLOC is defined during compilation, sallocx() and
malloc_usable_size() will still use ivsalloc().

This partially resolves #580.

INSTALL

@@ -104,7 +104,6 @@ any of the following arguments (not a definitive list) to 'configure':
 --enable-debug
     Enable assertions and validation code.  This incurs a substantial
     performance hit, but is very useful during application development.
-    Implies --enable-ivsalloc.
 
 --enable-code-coverage
     Enable code coverage support, for use during jemalloc test development.
@@ -123,12 +122,6 @@ any of the following arguments (not a definitive list) to 'configure':
     Disable statistics gathering functionality.  See the "opt.stats_print"
     option documentation for usage details.
 
---enable-ivsalloc
-    Enable validation code for malloc_usable_size() and sallocx(), which
-    verifies that pointers reside within jemalloc-owned extents before
-    dereferencing metadata.  This incurs a minor performance hit, and causes
-    the functions to return 0 for failed lookups.
-
 --enable-prof
     Enable heap profiling and leak detection functionality.  See the "opt.prof"
     option documentation for usage details.  When enabled, there are several