When clock_gettime(CLOCK_MONOTONIC) (or faster variants, _FAST on FreeBSD,
and _COARSE on Linux) is available, we now use it for ngx_current_msec.
This should improve handling of timers if system time changes (ticket #189).

Specifically, it is now allowed to start with a variable expression with braces:
${name}.  The opening curly bracket in such a token was previously considered
the start of a new block.  Variables located anywhere else in a token worked
fine: foo${name}.

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.

On some systems, it's possible that reaper of orphaned processes is
set to something other than "init" process.  On such systems, the
changing binary procedure did not work.

The fix is to check if PPID has changed, instead of assuming it's
always 1 for orphaned processes.

This is what usually happens for zones no longer used in the new
configuration, but zones where size or tag were changed were freed
when creating new memory zones.  If reconfiguration failed (for
example, due to a conflicting listening socket), this resulted in a
segmentation fault in the master process.

Reported by Zhihua Cao,
http://mailman.nginx.org/pipermail/nginx-devel/2017-October/010536.html.

The type should have been changed in c9b243802a17 along with
changing ngx_conf_handler_pt.