rename follow option to --interval or -t

rename follow option to --interval or -t

Initially copied f(ollow) from tail, but that letter will be better suited
for field selection, and the function is only similar.

Alternatives include -s (closer equivalent to tail, also in free --seconds),
-i as in ping --interval (best match, but should be reserved for interactive
or ignore-case), -n like watch --interval, and -v as verbose interpretation.
Settle on -t for best mnemonics; common for time(out) and found in preferred
longhand of inTerval.