On Linux starting with 2.6.16, sendfile() silently limits all operations
to MAX_RW_COUNT, defined as (INT_MAX & PAGE_MASK).  This incorrectly
triggered the interrupt check, and resulted in 0-sized writev() on the
next loop iteration.

Fix is to make sure the limit is always checked, so we will return from
the loop if the limit is already reached even if number of bytes sent is
not exactly equal to the number of bytes we've tried to send.

This allows to build nginx on macOS with -Wdeprecated-declarations.

The ngx_wsasend_chain() and ngx_wsarecv_chain() functions were
modified to use only preallocated memory, and the number of
preallocated wsabufs was increased to 64.

In d1bde5c3c5d2, the number of preallocated iovec's for ngx_readv_chain()
was increased.  Still, in some setups, the function might allocate memory
for iovec's from a connection pool, which is only freed when closing the
connection.

The ngx_readv_chain() function was modified to use only preallocated
memory, similarly to the ngx_writev_chain() change in 8e903522c17a.

Due to structure's alignment, some uninitialized memory contents may have
been passed between processes.

Zeroing was removed in 0215ec9aaa8a.

Reported by Johnny Wang.

The strerrordesc_np() function, introduced in glibc 2.32, provides an
async-signal-safe way to obtain error messages.  This makes it possible
to avoid copying error messages.

Previously, systems without sys_nerr (or _sys_nerr) were handled with an
assumption that errors start at 0 and continuous.  This is, however, not
something POSIX requires, and not true on some platforms.

Notably, on Linux, where sys_nerr is no longer available for newly linked
binaries starting with glibc 2.32, there are gaps in error list, which
used to stop us from properly detecting maximum errno.  Further, on
GNU/Hurd errors start at 0x40000001.

With this change, maximum errno detection is moved to the runtime code,
now able to ignore gaps, and also detects the first error if needed.
This fixes observed "Unknown error" messages as seen on Linux with
glibc 2.32 and on GNU/Hurd.

Clearing cache based on free space left on a file system is
expected to allow better disk utilization in some cases, notably
when disk space might be also used for something other than nginx
cache (including nginx own temporary files) and while loading
cache (when cache size might be inaccurate for a while, effectively
disabling max_size cache clearing).

Based on a patch by Adam Bambuch.