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.

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.

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 PCRE is disabled, captures were treated as normal variables in
ngx_http_script_compile(), while code calculating flushes array length in
ngx_http_compile_complex_value() did not account captures as variables.
This could lead to write outside of the array boundary when setting
last element to -1.

Found with AddressSanitizer.

This is a port of corresponding http code with unrelated features excluded.