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/lib/libast/comp/conf.sh

@@ -333,7 +333,17 @@ $cc -E $tmp.c >/dev/null 2>&1 || systeminfo=
 
 CONF_getconf=
 CONF_getconf_a=
-for d in /usr/bin /bin /usr/sbin /sbin
+IFS=':'; set -f
+for d in \
+	/run/current-system/sw/bin \
+	/usr/xpg7/bin \
+	/usr/xpg6/bin \
+	/usr/xpg4/bin \
+	/usr/bin \
+	/bin \
+	/usr/sbin \
+	/sbin \
+	$PATH
 do	if	test -x $d/getconf
 	then	case `$d/getconf --?-version 2>&1` in
 		*"AT&T"*"Research"*)
@@ -348,6 +358,7 @@ do	if	test -x $d/getconf
 		break
 	fi
 done
+IFS=$ifs; set +f
 export CONF_getconf CONF_getconf_a
 
 case $verbose in