command -p should enable builtins not in path

[Vincent Lefevre <vincent@vinc17.net>]

Though zsh isn't meant to conform to POSIX, it should follow its
requirements when they make sense.

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html

says:

  The following options shall be supported:

  -p
    Perform the command search using a default value for PATH that is
    guaranteed to find all of the standard utilities.
                       ^^^

So, in particular, the standard utility "cd" must be found. Note that
the above sentence does not mean that the utility must be somewhere
in $PATH, just that the used PATH value allows the shell to find the
utility; in case of a builtin utility (like "cd"), this will just run
the utility without needing PATH.

This works with all usual shells, except zsh:

zira% sh -c "command -p cd /tmp; pwd"
/tmp
zira% bash -c "command -p cd /tmp; pwd"
/tmp
zira% ksh -c "command -p cd /tmp; pwd"
/tmp
zira% zsh -c "command -p cd /tmp; pwd"
zsh:1: command not found: cd
/home/vinc17
zira% echo $ZSH_VERSION
5.8

Setting the POSIX_BUILTINS option allows "cd" to be found, but with
the drawback that it will not disable builtins that are in $PATH.
So this option is a bad solution when using "zmodload zsh/files"
without -F, for instance.

Note: "command -p cd ..." is used by Intel's script to set up
environment variables for its compiler.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)