Reinstitate send() after syslogd restarts.

In r228193 the test of CONNPRIV have been moved to before the _usleep
and send in vsyslog().  When syslogd restarts, this would prevent the
message being logged after the disconnect/connect dance for
scenario #1.

PR:		194751
Submitted by:	Peter Creath <pjcreath+freebsd gmail com>
Reviewed By:	glebius
MFC after:	2 weeks
Differential Revision: https://reviews.freebsd.org/D1227
In preparation for using clang's -Wcast-qual:

Use __DECONST (instead of my own attempted re-invention) for the iov
parameters to jail_get/set(2).  Similarly remove the decost-ish hack
from execvp's argv, except the __DECONST is only added at very end.

While I'm at it, remove an unused variable and fix a comment typo.