adding '-p precmd' to _normal

[Jun T <takimoto-j@xxxxxxxxxxxxxxxxx>]

In workers/52037⁩, I replaced '_normal' by '_normal -p env' 
in _env (and similarly in _watch), so that only external
command (and its args) are completed.

But I've noticed there are many (about 15?) completion
functions that should complete only external commands
and may require the similar change (adding -p precmd).
Examples are _chroot, _chrt, _screen, etc. 

In some other completion functions (_dchroot, _truss, etc.)
'_command_name -e' is used for completing external
command and '_normal' is used for args.

If anyone knows any reason that this two-step method is
better than '_normal -p precmd' then please let me know.

Otherwise I will use '_normal -p precmd' in _chroot etc.