[BUG] 'trap' is not quite POSIX compliant

[Martijn Dekker <martijn@xxxxxxxx>]

The 'trap' command in dash is not compliant with POSIX. According to the
spec, both '-' and any unsigned decimal integer should be accepted as an
argument meaning 'unset this trap'; dash only accepts '-'.
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_28

I did testing on all other POSIX shells I know of with this little script:

	trap 'echo BYYYE' EXIT
	trap "$1" EXIT

This script will either produce 'BYYYE' or output a "command not found"
error message, depending on whether $1 was accepted as an argument for
unsetting the trap.

None of the shells are technically compliant: they only interpret an
unsigned decimal integer up to a certain value as 'unset this trap'; a
value exceeding that is interpreted as a command.

Interestingly, that maximum value differs slightly between shells. So
far, I've found:
- dash and Busybox ash do not accept any.
- bash, yash, pdksh, mksh/lksh, FreeBSD /bin/sh, and AT&T ksh "AJM 93u+
2012-08-01" accept up to 31.
- AT&T ksh "M 1993-12-28 s+" accepts up to 32.
- zsh 4.3.11, zsh 5.1.1 and zsh 5.2-dev-1 (current git) on my Mac accept
up to 33.
- zsh 5.0.2 on FreeBSD accepts up to 34.

For compatibility purposes it might seem wise to follow the majority of
implementations, accepting up to 31.

By the way, dash and zsh, as well as bash in non-POSIX mode, also accept
the counterintuitive and non-POSIX-compliant form "trap EXIT" to unset
the trap, but other shells don't: bash (POSIX), yash and ksh93 produce
an error, while mksh/lksh silently ignore that form. What a mess...

- M.
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