Merge of r4778, r4782, r4783, r4824, r4830, r4834: minor fixes.

Merge of r4778, r4782, r4783, r4824, r4830, r4834: minor fixes.

*) Reorder checks in ngx_shared_memory_add() for more consistent
   error messages.

*) Added "const" to ngx_memcpy() with NGX_MEMCPY_LIMIT defined.  This
   fixes warning produced during compilation of the ngx_http_geoip_module
   due to const qualifier being discarded.

*) Fixed possible use of old cached times if runtime went backwards.

   If ngx_time_sigsafe_update() updated only ngx_cached_err_log_time, and
   then clock was adjusted backwards, the cached_time[slot].sec might
   accidentally match current seconds on next ngx_time_update() call,
   resulting in various cached times not being updated.

   Fix is to clear the cached_time[slot].sec to explicitly mark cached times
   are stale and need updating.

*) Radix tree preallocation fix.  The preallocation size was calculated
   incorrectly and was always 8 due to sizeof(ngx_radix_tree_t) accidentally
   used instead of sizeof(ngx_radix_node_t).

*) Fixed overflow if ngx_slab_alloc() is called with very big "size"
   argument.

*) Write filter: replaced unneeded loop with one to free chains.
   Noted by Gabor Lekeny.