Log Message:
Move the check-shlibs target from bsd.pkg.mk to bsd.pkg.check.mk where
it will live with other "check" targets run after package installation.

Get rid of SHLIB_HANDLING, whose meaning had mutated over the years
from one thing to another.  Currently, it is used to basically note
whether the system's "ldd" command can be usefully run on the package's
binaries and libraries.  Rename this variable to CHECK_SHLIBS_SUPPORTED
for more clarity.

CHECK_SHLIBS is now a variable set exclusively by the user in /etc/mk.conf
to note whether the check for missing run-time search paths is performed
after a package is installed.  It defaults to "no" unless PKG_DEVELOPER
is set.