Fixes to compile on Solaris variants, NetBSD, and NixOS

Solaris, Illumos distributions, and NetBSD need LDFLAGS set to link
explicitly to libm, otherwise, due to as-yet unknown reasons, the
src/lib/libdll/features/dll fails to write a valid header file and
compilation fails due to unknown identifiers such as Dllscan_t.
This commit adds the flag on those systems.

NixOS is a Linux distro that uses very different paths from the
usual Unix conventions (though it's POSIX compliant), and the
regression tests still needed a lot of tweaks to be compatible.

src/cmd/INIT/package.sh, bin/package:
- On SunOS (Solaris and illumos distros) and NetBSD, add '-lm' to
  LDFLAGS before compiling.

src/cmd/INIT/mamprobe.sh, bin/mamprobe,
src/cmd/INIT/execrate.sh, bin/execrate:
- Instead of only in /bin, /usr/bin, /sbin and /usr/sbin, search
  utilities in the path given by the OS 'getconf PATH', and use the
  user's original $PATH as a fallback.

src/cmd/ksh93/tests/*.sh:
- Miscellaneous portability fixes, mainly elimination of unportable
  hardcoded paths to commands.
- basic.sh: Remove test for 'time' keyword millisecond precision.
  It was racy and could fail depending on system and system load.

src/cmd/ksh93/tests/variables.sh

@@ -745,7 +745,7 @@ do	exp="$cmd ok"
 	rm -f $cmd
 	print print $exp > glean/$cmd
 	chmod +x glean/$cmd
-	got=$(set +x; CDPATH=:.. $SHELL -c "PATH=:/bin:/usr/bin; date > /dev/null; cd glean && ./$cmd" 2>&1)
+	got=$(set +x; CDPATH=:.. $SHELL -c "command -p date > /dev/null; cd glean && ./$cmd" 2>&1)
 	[[ $got == "$exp" ]] || err_exit "cd with CDPATH after PATH change failed -- expected '$exp', got '$got'"
 done