Improve interactiveness checks (#3449)

* [bash] return instead of not executing an if-block, when non-interactive

This should keep the code more readable, be less error prone (accidentally doing
something outside the if-block and aligns the code with what’s already done for
zsh.

`0` is returned, because it shall not be considered an error when the script is
(accidentally) sourced from a non-interactive shell.

If executed as a script (rather than sourced), the results are not specified by
POSIX but depend on the shell, with bash giving an error in that case.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>

* [shell] exit immediately when called from non-interactive shell

The shell execution environment shouldn’t be modified at all, when called from a
non-interactive shell.

It shall be noted that the current check may become error prone for bash, namely
in case there should ever be a differentiation between `i` and `I` in the
special variable `-` and bash’s `nocasematch`-shell-option be used.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>

shell/key-bindings.zsh

@@ -11,6 +11,9 @@
 # - $FZF_ALT_C_COMMAND
 # - $FZF_ALT_C_OPTS
 
+[[ -o interactive ]] || return 0
+
+
 # Key bindings
 # ------------
 
@@ -36,8 +39,6 @@ fi
 
 {
 
-[[ -o interactive ]] || return 0
-
 # CTRL-T - Paste the selected file path(s) into the command line
 __fsel() {
   local cmd="${FZF_CTRL_T_COMMAND:-"command find -L . -mindepth 1 \\( -path '*/.*' -o -fstype 'sysfs' -o -fstype 'devfs' -o -fstype 'devtmpfs' -o -fstype 'proc' \\) -prune \