bectl: make mount subcommand less verbose

The mount subcommand currently produces output such as:
    # bectl mount <bootenv>
    Successfully mounted <bootenv> at <mountpoint>

This commit changes it to only print the mountpoint:
    # bectl mount <bootenv>
    <mountpoint>

This makes it easier to script the mount subcommand. If an error occurs
while mounting, an error message is printed to stderr and bectl will
exit with a non-zero value.

PR:             273180
Reviewed by:    kevans, asomers
Differential Revision:	https://reviews.freebsd.org/D41562

sbin/bectl/bectl.c

@@ -440,7 +440,7 @@ bectl_cmd_mount(int argc, char *argv[])
 
 	switch (err) {
 	case BE_ERR_SUCCESS:
-		printf("Successfully mounted %s at %s\n", bootenv, result_loc);
+		printf("%s\n", result_loc);
 		break;
 	default:
 		fprintf(stderr,