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/shtests

@@ -256,7 +256,10 @@ case $SHELL in
 */*)	SHELL=$d/$SHELL;;
 *)	SHELL=$(whence $SHELL);;
 esac
-PATH=/bin:/usr/bin
+PATH=$(
+	PATH=/run/current-system/sw/bin:/usr/xpg7/bin:/usr/xpg6/bin:/usr/xpg4/bin:/bin:/usr/bin:$PATH
+	getconf PATH 2>/dev/null || { builtin getconf 2>/dev/null && getconf PATH; }
+) || PATH=/bin:/usr/bin
 if	[[ -d /usr/ucb ]]
 then	PATH=$PATH:/usr/ucb
 fi