Ticket #3125: don't override precmd on Zsh

Do not override global precmd() funcation on Zsh. Define new _mc_precmd()
function and add it to precmd_functions array instead. This also effectively
fixes race condition with pwd call on Zsh, which may lead to empty prompt in mc.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Merge branch '3125_zsh_precmd'

* 3125_zsh_precmd:
  Ticket #3125: don't override precmd on Zsh