Fix some long-standing malloc bugs in the package handling code (freeing garbage

Fix some long-standing malloc bugs in the package handling code (freeing garbage
in one place, leaking memory in another).

Add a facility to invoke subsystems directly by naming them on
sysinstall's command-line when running post-install.  A replacement
for pkg_manage might, for example, be `/stand/sysinstall configPackages'

Fix bogon where upgrade shell was entered with tty modes spammed.

Fix bug with release name checking in ftp_strat.  Turned a bunch of
bogus exit()s into proper calls to systemShutdown().