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/cmd/ksh93/features/math.sh

@@ -22,6 +22,11 @@
 
 # @(#)math.sh (AT&T Research) 2012-06-13
 
+case $ZSH_VERSION in
+?*)	emulate ksh ;;
+*)	(set -o posix) 2>/dev/null && set -o posix ;;
+esac
+
 command=$0
 iffeflags="-n -v"
 iffehdrs="math.h"