Always pass the test program as PROG env var, fix synopsis

The ./th perl script always runs -p prog (KSH in Makefile) as the shell,
but cannot signal it other than the optional SIGKILL after `time-limit'.

Implementing time-related signal handling in ./th seems overkill, so always
pass the program as PROG in the environment so it can wrap itself, e.g.
	description:
		simluate interrupting a script
	stdin:
		timeout --signal SIGINT --preserve-status -- 1s $PROG -c '
			actual test script getting ^C after one second...
		'

Avoiding hard-coded programs in tests with PROG allows testing different
shells through `make KSH=/path/to/sh' without changing or adding shell
specific tests.

While here, make -p and -s as mandatory in usage as ./th `die's if they're
not passed (allowing to always just pass -p's argument as PROG).

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