AIO support in nginx was originally developed against FreeBSD versions 4-6,
where the sival_ptr field was named as sigval_ptr (seemingly by mistake[1]),
which made nginx use the only name available then.  The standard-complaint
name was restored in 2005 (first appeared in FreeBSD 7.0, 2008), retaining
compatibility with previous versions[2][3].  In DragonFly, similar changes
were committed in 2009[4], with backward compatibility recently removed[5].

The change switches to the standard name, retaining compatibility with old
FreeBSD versions.

[1] https://svnweb.freebsd.org/changeset/base/48621
[2] https://svnweb.freebsd.org/changeset/base/152029
[3] https://svnweb.freebsd.org/changeset/base/174003
[4] https://gitweb.dragonflybsd.org/dragonfly.git/commit/3693401
[5] https://gitweb.dragonflybsd.org/dragonfly.git/commit/7875042

Previous interface of ngx_open_dir() assumed that passed directory name
has a room for NGX_DIR_MASK at the end (NGX_DIR_MASK_LEN bytes).  While all
direct users of ngx_dir_open() followed this interface, this also implied
similar requirements for indirect uses - in particular, via ngx_walk_tree().

Currently none of ngx_walk_tree() uses provides appropriate space, and
fixing this does not look like a right way to go.  Instead, ngx_dir_open()
interface was changed to not require any additional space and use
appropriate allocations instead.

Previously, "%uA" was used, which corresponds to ngx_atomic_uint_t.
Size of ngx_atomic_uint_t can be easily different from uint64_t,
leading to undefined results.

The bug in question was fixed in glibc 2.3.2 and is no longer expected
to manifest itself on real servers.  On the other hand, the workaround
causes compilation problems on various systems.  Previously, we've
already fixed the code to compile with musl libc (fd6fd02f6a4d), and
now it is broken on Fedora 28 where glibc's crypt library was replaced
by libxcrypt.  So the workaround was removed.

No functional changes.

Previously, capset(2) was called with the 64-bit capabilities version
_LINUX_CAPABILITY_VERSION_3.  With this version Linux kernel expected two
copies of struct __user_cap_data_struct, while only one was submitted.  As a
result, random stack memory was accessed and random capabilities were requested
by the worker.  This sometimes caused capset() errors.  Now the 32-bit version
_LINUX_CAPABILITY_VERSION_1 is used instead.  This is OK since CAP_NET_RAW is
a 32-bit capability (CAP_NET_RAW = 13).

Previously included file sys/capability.h mentioned in capset(2) man page,
belongs to the libcap-dev package, which may not be installed on some Linux
systems when compiling nginx.  This prevented the capabilities feature from
being detected and compiled on that systems.

Now linux/capability.h system header is included instead.  Since capset()
declaration is located in sys/capability.h, now capset() syscall is defined
explicitly in code using the SYS_capset constant, similarly to other
Linux-specific features in nginx.

The capability is retained automatically in unprivileged worker processes after
changing UID if transparent proxying is enabled at least once in nginx
configuration.

The feature is only available in Linux.

Determine cacheline size at runtime if supported
using sysconf(_SC_LEVEL1_DCACHE_LINESIZE). In case not supported,
fallback to compile time defaults.