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.

bin/package

@@ -5395,6 +5395,19 @@ license)# all work in $PACKAGESRC/LICENSES
 	;;
 
 make|view)
+	# Hack to build on some systems that need an explicit link with libm due to a bug in the build system
+	case `uname` in
+	NetBSD | SunOS)
+		case " $LDFLAGS " in
+		*" -m "*)
+			;;
+		*)	LDFLAGS="-lm${LDFLAGS:+ $LDFLAGS}"
+			export LDFLAGS
+			;;
+		esac
+		;;
+	esac
+
 	cd $PACKAGEROOT
 	case $package in
 	'')	lic="lib/package/*.lic"