ksh compatibility: initial value of $_

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

Docs for ksh93 say that $_ should be initialized to the absolute path
of what we put in $ZSH_ARGZERO.

Src/init.c explicitly initializes $_ to the empty string.

Any reason not to change this?  If so, where's the right place to do
it?  I tried doing it in createparamtab() but that led to memory
errors.