Fix pass zsh completion and autoloading

When autocompleting from `pass <TAB>', sometimes the following errors
appear:

  _values:compvalues:10: not enough arguments
  find: `/home/user/.password-store': No such file or directory
  _values:compvalues:10: not enough arguments
  find: `/home/user/.password-store': No such file or directory

The `_values' error happens when there is no password-store folder *or*
there are no passwords in pass; the `find' error only when there is no
password-store folder.

We can trace it back to line 108, which contains the only `_values'
statement that is executed when we autocomplete from pass. We confirm
this by following the trail of execution, which is

  _pass -> _pass_cmd_show -> _pass_complete_entries ->
        -> _pass_complete_entries_helper

If we try running the command inside `$()' on line 104, we see that it
returns nothing and the output is blank. This means that `_values' only
receives 1 of its 2 mandatory parameters, therefore the above error is
triggered (not enough arguments).

That is unless we don't have a password-store folder, in which case the
`find: [...] no such file or directory' error is *also* triggered.

We solve the first error by supplying a default value of "" if the
command outputs nothing, using the zsh construct ${var:-else}.

We solve the second error by redirecting the find command's stderr output
to /dev/null, so the error is effectively suppressed.

* * * *

This patch also fixes the first tab completion, which currently only
loads the completion function definition.

We do this by adding a `_pass' statement at the end of the file, which
runs the `_pass' completion function after loading its definition.
This is the standard way an autoloaded function works; for other examples
look at zsh's official completion files.