No functional changes.

log->filter ("if" parameter) was uninitialized when the default value
was being used, which would lead to a crash (SIGSEGV) when access_log
directive wasn't specified in the configuration.

Zero-fill the whole structure instead of zeroing fields one-by-one
in order to prevent similar issues in the future.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>

Large allocations from a slab pool result in free page blocks being fragmented,
eventually leading to a situation when no further allocation larger than a page
size are possible from the pool.  While this isn't a problem for nginx itself,
it is known to be bad for various 3rd party modules.  Fix is to merge adjacent
blocks of free pages in the ngx_slab_free_pages() function.

Prodded by Wandenberg Peixoto and Yichun Zhang.

Since the type cast has precedence higher than the bit shift operator,
all values were truncated to 8 bits.

These macros are used to construct header block for SYN_REPLY frame on
platforms with strict alignment requirements.  As a result, any response
that contains a header with name or value longer than 255 bytes was
corrupted on such platforms.

Do not taste the last parameter against directory, as otherwise it would
result in the trailing slash being cut from the parameter value.

Notably, this prevents an internal redirect to an empty URI
if the parameter is set to the literal slash:

    location / { try_files $uri /; }

Previous workaround to avoid warnings on OS X due to deprecated system
OpenSSL library (introduced in a3870ea96ccd) no longer works, as
the MAC_OS_X_VERSION_MIN_REQUIRED macro is ignored on OS X 10.9
if a compiler used supports __attribute__(availability).