Convert "have_prog" function into "which".

"which" and its behaviour is not standardized, so convert the existing
have_prog function into "which" so we can rely on it being available
and what its semantics are.  Add a have_prog wrapper that maintains the
existing behaviour.

regress/test-exec.sh

@@ -320,7 +320,7 @@ export SSH_PKCS11_HELPER SSH_SK_HELPER
 #echo $SSH $SSHD $SSHAGENT $SSHADD $SSHKEYGEN $SSHKEYSCAN $SFTP $SFTPSERVER $SCP
 
 # Portable specific functions
-have_prog()
+which()
 {
 	saved_IFS="$IFS"
 	IFS=":"
@@ -328,13 +328,21 @@ have_prog()
 	do
 		if [ -x $i/$1 ]; then
 			IFS="$saved_IFS"
+			echo "$i/$1"
 			return 0
 		fi
 	done
 	IFS="$saved_IFS"
+	echo "$i/$1"
 	return 1
 }
 
+have_prog()
+{
+	which "$1" >/dev/null 2>&1
+	return $?
+}
+
 jot() {
 	awk "BEGIN { for (i = $2; i < $2 + $1; i++) { printf \"%d\n\", i } exit }"
 }