The h->next pointer properly provided as NULL in all cases where known
output headers are added.

Note that there are 3rd party modules which might not do this, and it
might be risky to rely on this for arbitrary headers.

With this change, it is now possible to use ngx_conf_merge_ptr_value()
to merge complex values.  This change follows much earlier changes in
ngx_conf_merge_ptr_value() and ngx_conf_set_str_array_slot()
in 1452:cd586e963db0 (0.6.10) and 1701:40d004d95d88 (0.6.22), and the
change in ngx_conf_set_keyval_slot() (7728:485dba3e2a01, 1.19.4).

To preserve compatibility with existing 3rd party modules, both NULL
and NGX_CONF_UNSET_PTR are accepted for now.

The "alias" directive cannot be used in the same location where URI
was rewritten.  This has been detected in the "rewrite ... break"
case, but not when the standalone "break" directive was used.

This change also fixes proxy_pass with URI component in a similar
case:

       location /aaa/ {
           rewrite ^ /xxx/yyy;
           break;
           proxy_pass http://localhost:8080/bbb/;
       }

Previously, the "/bbb/yyy" would be sent to a backend instead of
"/xxx/yyy".  And if location's prefix was longer than the rewritten
URI, a segmentation fault might occur.

If a complex value is expected to be of type size_t, and the compiled
value is constant, the constant size_t value is remembered at compile
time.

The value is accessed through ngx_http_complex_value_size() which
either returns the remembered constant or evaluates the expression
and parses it as size_t.

In contrast to ngx_http_test_predicates(), it requires all values to be
non-empty and not equal to "0".

Cast to intermediate "void *" to lose compiler knowledge about the original
type and pass the warning.  This is not a real fix but rather a workaround.

Found by gcc8.

If of.err is 0, it means that there was a memory allocation error
and no further logging and/or processing is needed.  The of.failed
string can be only accessed if of.err is not 0.

There are lots of C4244 warnings (conversion from 'type1' to 'type2',
possible loss of data), so they were disabled.

The same applies to C4267 warnings (conversion from 'size_t' to 'type',
possible loss of data), most notably - conversion from ngx_str_t.len to
ngx_variable_value_t.len (which is unsigned:28).  Additionally, there
is at least one case when it is not possible to fix the warning properly
without introducing win32-specific code: recv() on win32 uses "int len",
while POSIX defines "size_t len".

The ssize_t type now properly defined for 64-bit compilation with MSVC.
Caught by warning C4305 (truncation from '__int64' to 'ssize_t'), on
"cutoff = NGX_MAX_SIZE_T_VALUE / 10" in ngx_atosz()).

Several C4334 warnings (result of 32-bit shift implicitly converted to 64 bits)
were fixed by adding explicit conversions.

Several C4214 warnings (nonstandard extension used: bit field types other
than int) in ngx_http_script.h fixed by changing bit field types from
uintptr_t to unsigned.