zsh-completion: regression when using BSD sed

Made sed expression GNU and BSD compatible.

A recent change in pass.zsh-completion broke autocompletion when using
BSD sed. I’ve made the relevant sed expression compatible with GNU and
BSD sed.

Previous change with regression:
http://lists.zx2c4.com/pipermail/password-store/2014-April/000773.html

http://git.zx2c4.com/password-store/commit/src/completion/pass.zsh-completion?id=f82e9d6cf3bc3a12bdfce89bf319d76f79e66efc
Mute git-commit messages to make pass insert readable

`pass generate' prints 10 lines of output, with only 2 related
to the operation at hand:
% pass generate xyz
1
2  You need a passphrase to unlock the secret key for
3  user: USER <EMAIL>
4  4096-bit RSA key, ID XYZ, created 1234-56-78
5
6  [master 12345] Added generated password for xyz to store.
7  1 file changed, 0 insertions(+), 0 deletions(-)
8   create mode 100644 xyz.gpg
9  The generated password to xyz is:
10 Some password 123
%

This patch takes care of 3 of 8 noise lines.
Import new kwallet importer.