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.

In 2c7b488a61fb, IP_BIND_ADDRESS_NO_PORT test was accidentally placed
between SO_BINDANY, IP_TRANSPARENT, and IP_BINDANY tests.  Moved it after
these tests.

As per POSIX, basic regular expressions have no alternations, and the
interpretation of the "\|" construct is undefined.  At least on MINIX
and Solaris grep interprets "\|" as literal "|", and not as an alternation
as GNU grep does.  Removed such constructs introduced in f1daa0356a1d.
This fixes clang detection on MINIX.

The phase is added instead of the try_files phase.  Unlike the old phase, the
new one supports registering multiple handlers.  The try_files implementation is
moved to a separate ngx_http_try_files_module, which now registers a precontent
phase handler.