Degrade gracefully on systems missing zsh/system module

When using async mode, stale background processes will not be cancelled
when a new one starts. This shouldn't cause any real issues since the
processes should eventually finish and be cleaned up anyway, and
removing the handler with `zle -F` means that stale suggestions should
never be shown.

src/async.zsh

@@ -3,9 +3,9 @@
 # Async                                                              #
 #--------------------------------------------------------------------#
 
-zmodload zsh/system
-
 _zsh_autosuggest_async_request() {
+	zmodload zsh/system 2>/dev/null # For `$sysparams`
+
 	typeset -g _ZSH_AUTOSUGGEST_ASYNC_FD _ZSH_AUTOSUGGEST_CHILD_PID
 
 	# If we've got a pending request, cancel it
@@ -14,6 +14,8 @@ _zsh_autosuggest_async_request() {
 		exec {_ZSH_AUTOSUGGEST_ASYNC_FD}<&-
 		zle -F $_ZSH_AUTOSUGGEST_ASYNC_FD
 
+		# We won't know the pid unless the user has zsh/system module installed
+		if [[ -n "$_ZSH_AUTOSUGGEST_CHILD_PID" ]]; then
 			# Zsh will make a new process group for the child process only if job
 			# control is enabled (MONITOR option)
 			if [[ -o MONITOR ]]; then
@@ -27,6 +29,7 @@ _zsh_autosuggest_async_request() {
 				kill -TERM $_ZSH_AUTOSUGGEST_CHILD_PID 2>/dev/null
 			fi
 		fi
+	fi
 
 	# Fork a process to fetch a suggestion and open a pipe to read from it
 	exec {_ZSH_AUTOSUGGEST_ASYNC_FD}< <(

zsh-autosuggestions.zsh

@@ -726,9 +726,9 @@ _zsh_autosuggest_fetch_suggestion() {
 # Async                                                              #
 #--------------------------------------------------------------------#
 
-zmodload zsh/system
-
 _zsh_autosuggest_async_request() {
+	zmodload zsh/system 2>/dev/null # For `$sysparams`
+
 	typeset -g _ZSH_AUTOSUGGEST_ASYNC_FD _ZSH_AUTOSUGGEST_CHILD_PID
 
 	# If we've got a pending request, cancel it
@@ -737,6 +737,8 @@ _zsh_autosuggest_async_request() {
 		exec {_ZSH_AUTOSUGGEST_ASYNC_FD}<&-
 		zle -F $_ZSH_AUTOSUGGEST_ASYNC_FD
 
+		# We won't know the pid unless the user has zsh/system module installed
+		if [[ -n "$_ZSH_AUTOSUGGEST_CHILD_PID" ]]; then
 			# Zsh will make a new process group for the child process only if job
 			# control is enabled (MONITOR option)
 			if [[ -o MONITOR ]]; then
@@ -750,6 +752,7 @@ _zsh_autosuggest_async_request() {
 				kill -TERM $_ZSH_AUTOSUGGEST_CHILD_PID 2>/dev/null
 			fi
 		fi
+	fi
 
 	# Fork a process to fetch a suggestion and open a pipe to read from it
 	exec {_ZSH_AUTOSUGGEST_ASYNC_FD}< <(