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/basic.sh

@@ -553,21 +553,7 @@ $SHELL 2> /dev/null -c $'for i;\ndo :;done' || err_exit 'for i ; <newline> not v
 ) || err_exit "crash when sourcing multiple files (exit status $?)"
 
 # ======
-# The time keyword should correctly handle millisecond precision, except
-# on older systems that do not support getrusage(2) or gettimeofday(2).
-result=$(
-	TIMEFORMAT=$'\%3R'
-	redirect 2>&1
-	set +x
-	time sleep .002
-)
-case $result in
-0.000 | 0.010)
-	err_exit "warning: 'time' keyword doesn't support millisecond precision"
-	let "Errors--" ;;
-0.002)	;;
-*)	err_exit "'time' keyword, millisecond precision: expected 0.002, got $result" ;;
-esac
+# The time keyword
 
 # A single '%' after a format specifier should not be a syntax
 # error (it should be treated as a literal '%').