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/attributes.sh

@@ -492,14 +492,13 @@ typeset -A expect=(
 	[Mtoupper]='typeset -x -r -u foo'
 	[R]='typeset -x -r -R 0 foo'
 #	[R17]='typeset -x -r -R 17 foo'		# TODO: outputs '-L 0'
-	[X]='typeset -x -r -X 32 foo'
 	[X17]='typeset -x -r -X 17 foo'
 	[S]='typeset -x -r foo'
 	[T]='typeset -x -r foo'
 	[Z]='typeset -x -r -Z 0 -R 0 foo'
 #	[Z13]='typeset -x -r -Z 13 -R 13 foo'	# TODO: outputs 'typeset -x -r -Z 0 -R 0 foo'
 )
-for flag in a b i i37 l n s u A C E E12 F F12 H L Mtolower Mtoupper R X X17 S T Z
+for flag in a b i i37 l n s u A C E E12 F F12 H L Mtolower Mtoupper R X17 S T Z
 do	unset foo
 	actual=$(
 		redirect 2>&1