make not blocking signals the default

This is the next step in determining whether we can disable blocking
signals without a good reason to do so. This makes not blocking signals
the default behavior. If someone finds a problem they can add this to
their ~/config/fish/config.fish file:

set FISH_NO_SIGNAL_BLOCK 0

Alternatively set that env var before starting fish. I won't be surprised
if people report problems. Till now we have relied on people opting in
to this behavior to tell us whether it causes problems. This makes the
experimental behavior the default that has to be opted out of. This will
give us a lot more confidence this change doesn't cause problems before
the next minor release.

Note that there are still a few places where we force blocking of
signals. Primarily to keep SIGTSTP from interfering with the shell in
response to manipulating the controlling tty. Bash is more selective
in the signals it blocks around the problematic syscalls (c.f., its
`git_terminal_to()` function). However, I don't see any value in that
refinement.

src/fish.cpp

@@ -368,7 +368,9 @@ int main(int argc, char **argv) {
         // TODO: Remove this once we're confident that not blocking/unblocking every signal around
         // some critical sections is no longer necessary.
         env_var_t fish_no_signal_block = env_get_string(L"FISH_NO_SIGNAL_BLOCK");
-        if (!fish_no_signal_block.missing()) ignore_signal_block = true;
+        if (!fish_no_signal_block.missing_or_empty() && !from_string<bool>(fish_no_signal_block)) {
+            ignore_signal_block = false;
+        }
 
         // Stomp the exit status of any initialization commands (issue #635).
         proc_set_last_status(STATUS_BUILTIN_OK);