getty: code cleanup, part 1

getty: code cleanup, part 1

* Avoid unnecessary use of `unsigned char *`
* Use explicit casts when assigning `unsigned char *` to `char *` or vice versa
* Drop unused global variables (and fix memory leak in `gettable()`)
* Use `snprintf()` instead of `strcpy()` + `strcat()`
* Drop spurious braces in switch

Sponsored by:	Klara, Inc.
Obtained from:	Apple OSS Distributions (in part)
Differential Revision: https://reviews.freebsd.org/D37263

(cherry picked from commit 8ad7a14ab49b93240676e15f404354775be931f4)

getty: code cleanup, part 2

* Clean up whitespace
* Reindent

Sponsored by:	Klara, Inc.

(cherry picked from commit f285f41498ebe0bfc1dbe47d67af12ec2d131521)

getty: Avoid NULL deref if stdin is not a tty.

Sponsored by:	Klara, Inc.
Obtained from:	Apple OSS Distributions
Differential Revision: https://reviews.freebsd.org/D37265

(cherry picked from commit eb4d86d529e2523a19fd7454976923319954a49d)