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.

bin/mamprobe

@@ -21,9 +21,7 @@
 # mamprobe - generate MAM cc probe info
 # Glenn Fowler <gsf@research.att.com>
 
-case $-:$BASH_VERSION in
-*x*:[0123456789]*)	: bash set -x is broken :; set +ex ;;
-esac
+(set -o posix) 2>/dev/null && set -o posix
 
 command=mamprobe