build system: modernise shell compatibility checks

All changed files:
- Put the shell in POSIX mode if it has an '-o posix' option.
- Remove nonsense disabling 'set -x' on bash. It's not broken.

bin/package, src/cmd/INIT/package.sh:
- Add check blocking native zsh mode (e.g., "$path" conflicts).
  Using a 'sh -> zsh' symlink works, so recommend that.
- Remove old ksh93 version check for a supposed conflict with
  libcmd. It was broken; it would revert to /bin/sh, but on illumos
  distributions, /bin/sh is a ksh93 of a version that is supposedly
  affected. It builds fine anyway.
- Rewrite checksh() to incorporate the shell compatibility checks
  that were previously in two different places in 'package'.

bin/ignore, src/cmd/INIT/ignore.sh,
bin/silent, src/cmd/INIT/silent.sh:
- Change bad check for a full POSIX 'export' command (no, $RANDOM
  has nothing to do with that) with a proper feature test.

src/lib/libast/comp/conf.sh

@@ -34,8 +34,9 @@
 # but you shall be confused anyway
 #
 
-case $-:$BASH_VERSION in
-*x*:[0123456789]*)	: bash set -x is broken :; set +ex ;;
+case $ZSH_VERSION in
+?*)	emulate ksh ;;
+*)	(set -o posix) 2>/dev/null && set -o posix ;;
 esac
 
 LC_ALL=C

src/lib/libast/features/libpath.sh

@@ -19,6 +19,12 @@
 #                   Phong Vo <kpv@research.att.com>                    #
 #                                                                      #
 ########################################################################
+
+case $ZSH_VERSION in
+?*)	emulate ksh ;;
+*)	(set -o posix) 2>/dev/null && set -o posix ;;
+esac
+
 ok=0
 for i in \
 	-x /lib/ld.so /lib/ld-*.so /usr/lib/ld.so /lib/rld \

src/lib/libast/features/param.sh

@@ -20,6 +20,12 @@
 #                                                                      #
 ########################################################################
 : generate "<sys/param.h> + <sys/types.h> + <sys/stat.h>" include sequence
+
+case $ZSH_VERSION in
+?*)	emulate ksh ;;
+*)	(set -o posix) 2>/dev/null && set -o posix ;;
+esac
+
 case $# in
 0)	;;
 *)	eval $1

src/lib/libast/features/preroot.sh

@@ -20,6 +20,12 @@
 #                                                                      #
 ########################################################################
 : generate preroot features
+
+case $ZSH_VERSION in
+?*)	emulate ksh ;;
+*)	(set -o posix) 2>/dev/null && set -o posix ;;
+esac
+
 case $# in
 0)	;;
 *)	eval $1

src/lib/libast/features/sig.sh

@@ -20,6 +20,12 @@
 #                                                                      #
 ########################################################################
 : generate sig features
+
+case $ZSH_VERSION in
+?*)	emulate ksh ;;
+*)	(set -o posix) 2>/dev/null && set -o posix ;;
+esac
+
 case $# in
 0)	;;
 *)	eval $1