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/signal.sh

@@ -442,10 +442,10 @@ let "$? == 256+9" && err_exit 'exit with status > 256 makes shell kill itself'
 # and that a signal still causes the exit status to be set to a value > 128
 cat >"$tmp/sigtest.sh" <<\EOF
 echo begin
-sh -c 'kill -9 "$$"'
+"$1" -c 'kill -9 "$$"'
 EOF
 expect=$'^begin\n/.*/sigtest.sh: line 2: [1-9][0-9]*: Killed\n[1-9][0-9]{1,2}$'
-actual=$("$SHELL" -c 'ksh "$1"; echo "$?"' x "$tmp/sigtest.sh" 2>&1)
+actual=$(LANG=C "$SHELL" -c '"$1" "$2" "$1"; echo "$?"' x "$SHELL" "$tmp/sigtest.sh" 2>&1)
 if	! [[ $actual =~ $expect ]]
 then	[[ $actual == *Killed*Killed* ]] && msg='ksh killed itself' || msg='unexpected output'
 	err_exit "$msg after child process signal (expected match to $(printf %q "$expect"); got $(printf %q "$actual"))"