build system: fix detection of default system directories

src/cmd/INIT/iffe.sh:
- Fix "standard system directories" for the cmd test, which were
  hardcoded as bin, /etc, /usr/bin, /usr/etc, /usr/ucb. That's both
  unportable and antiquated. Replace this with the path output by
  'getconf PATH'.
- Add fixes from modernish for 'getconf PATH' output to compensate
  for bugs/shortcomigns in NixOS and AIX. Source:
  9e4bf5eb/lib/modernish/aux/defpath.sh
  Ref.: https://github.com/NixOS/nixpkgs/issues/65512

src/lib/libast/comp/conf.tab: PATH:
- Add the NixOS and AIX default path fixes here too; this fixes
  'command -p' and the builtin 'getconf PATH' on these systems.

bin/package, src/cmd/INIT/package.sh:
- Re-support being launched with just the command name 'package' in
  the command line (if the 'package' command is in $PATH). At least
  one other script in the build system does this. (re: 6cc2f6a0)
- Go back three levels (../../..) if we were invoked from
  arch/*/bin/package, otherwise we won't find src/cmd/ksh93/SHOPT.sh.

src/lib/libast/comp/conf.tab

@@ -375,9 +375,41 @@ PATH				AST	CS 1 MU		sh{
 	*)	CONF_path=`"$CONF_getconf" PATH` ;;
 	esac
 	case $CONF_path in
-	'')	echo '"/bin:/usr/bin:/sbin:/usr/sbin"' ;;
-	*)	echo "\\"$CONF_path\\"" ;;
+	'' | [!/]* | *:[!/]* | *: )
+		CONF_path=/bin:/usr/bin:/sbin:/usr/sbin ;;
 	esac
+	# Fix for NixOS. Not all POSIX standard utilities come with the default system,
+	# e.g. 'bc', 'file', 'vi'. The command that NixOS recommends to get missing
+	# utilities, e.g. 'nix-env -iA nixos.bc', installs them in a default profile
+	# directory that is not in $(getconf PATH). So add this path to the standard path.
+	# See: https://github.com/NixOS/nixpkgs/issues/65512
+	if	test -e /etc/NIXOS &&
+		nix_profile_dir=/nix/var/nix/profiles/default/bin &&
+		test -d "$nix_profile_dir"
+	then	case ":$CONF_path:" in
+		*:"$nix_profile_dir":* )
+			# nothing to do
+			;;
+		* )	# insert the default profile directory as the second entry
+			CONF_path=`
+				set -f
+				IFS=:
+				set $CONF_path
+				one=$1
+				shift
+				echo "$one:$nix_profile_dir${1+:}$*"
+			` ;;
+		esac
+	fi
+	# Fix for AIX. At least as of version 7.1, the system default 'find', 'diff -u' and 'patch' utilities
+	# are broken and/or non-compliant in ways that make them incompatible with POSIX 2018. However, GNU
+	# utilities are commonly installed in /opt/freeware/bin, and under standard names (no g- prefix).
+	if	test -d /opt/freeware/bin
+	then	case `uname` in
+		AIX )	CONF_path="/opt/freeware/bin:$CONF_path" ;;
+		esac
+	fi
+	echo "\\"$CONF_path\\""
 }
 PATH_MAX			POSIX	PC 1 CDLMUX	MAXPATHLEN 1024
 PBS				POSIX	SC 2 FUW