zfsbootmenu: protect stdout of main interface

fzf forces us to capture stdout to know which key combination was
pressed and which item was selected. A bug has been lurking for ages:

* Hit mod-l on the main screen to see the warning/error log
* Hit enter to select whatever is shown
* Hit mod-p to look at the pool status screen

When the sub fzf instance used to draw logs exits with a selection (e.g.
with the enter key), it writes that to stdout. Then when the lower level
fzf instance exits (e.g. the main menu screen) with a selection, stdout
from both instances is captured by the subshell.

This solution protects against that by discarding stdout from all of the
sub-instances of fzf.

zfsbootmenu/lib/fzf-defaults.sh

@@ -9,12 +9,12 @@
 fuzzy_default_options=(
   "--ansi" "--no-clear" "--cycle" "--color=16"
   "--layout=reverse-list" "--inline-info" "--tac"
-  "--bind" '"alt-h:execute[ /libexec/zfsbootmenu-help -L ${HELP_SECTION:-main-screen} ]"'
-  "--bind" '"ctrl-h:execute[ /libexec/zfsbootmenu-help -L ${HELP_SECTION:-main-screen} ]"'
-  "--bind" '"ctrl-alt-h:execute[ /libexec/zfsbootmenu-help -L ${HELP_SECTION:-main-screen} ]"'
-  "--bind" "\"alt-l:execute[ /bin/zlogtail ]${HAS_REFRESH:++refresh-preview}\""
-  "--bind" "\"ctrl-l:execute[ /bin/zlogtail ]${HAS_REFRESH:++refresh-preview}\""
-  "--bind" "\"ctrl-alt-l:execute[ /bin/zlogtail ]${HAS_REFRESH:++refresh-preview}\""
+  "--bind" '"alt-h:execute[ /libexec/zfsbootmenu-help -L ${HELP_SECTION:-main-screen} 1>/dev/null ]"'
+  "--bind" '"ctrl-h:execute[ /libexec/zfsbootmenu-help -L ${HELP_SECTION:-main-screen} 1>/dev/null ]"'
+  "--bind" '"ctrl-alt-h:execute[ /libexec/zfsbootmenu-help -L ${HELP_SECTION:-main-screen} 1>/dev/null ]"'
+  "--bind" "\"alt-l:execute[ /bin/zlogtail 1>/dev/null ]${HAS_REFRESH:++refresh-preview}\""
+  "--bind" "\"ctrl-l:execute[ /bin/zlogtail 1>/dev/null ]${HAS_REFRESH:++refresh-preview}\""
+  "--bind" "\"ctrl-alt-l:execute[ /bin/zlogtail 1>/dev/null ]${HAS_REFRESH:++refresh-preview}\""
 )
 
 if [ -n "${HAS_BORDER}" ]; then