script: Correct -F in man page and usage string.

Sponsored by: Klara, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37138

(cherry picked from commit 8e096e24d4344b6d3d16e24d58d5d8a0fa9733c9)

script: Add usage string for playback mode.

Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D37138

(cherry picked from commit e7c13cf4383d8216149becf7f843a2e00b30d2e2)

script: Use size_t / ssize_t where needed.

Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D37177

(cherry picked from commit 3276866f40caaf3d5605d5695f460cbdf035ef45)

script: Further usage string nits.

Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D37177

(cherry picked from commit 6ac087cf8ff4de22dc0b1b7ce1248f0a1558ddc2)

script: Handle a missing 's' stamp gracefully.

Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D37182

(cherry picked from commit 473e9fcab4ea9379f53e1f7d8e41f5a5754d0007)