Implement zsh shell completions.

zsh completions when gmx prefix was used were not implemented. Use zsh builtin
bashcompinit to use the bash completions. Remove 'shopt -s extglob'
from gmx-completion.bash and place it in GMXRC.bash, since it is incompatible
with zsh. gmx-completion.bash and gmx-completion-gmx.bash can now be used for
completions in both bash and zsh.

Change-Id: Ib8f3cf0535d39e91a6b31933f41aa7548755b351