Rewrite 3.6.1's compress_str() to avoid peeking past the end of the
input string. This should eliminate the reported valgrind complaint.
The problem was noticed for post-3.6.0 code introduced [by me...] last
June, but it looks like it was present in the old code too.
Also, fix the wording in the paragraph about NUL in the keyhelp text.
tty_putstr() always passes non-message window text through compress_str(),
clobbering usage of two spaces to separate sentences. putstr()'s caller
ought to have more control over that (possibly via its hardly ever used
'attribute' arg?).
d606b2a8ff