Hello,

I found that sygprocmask system call is being called lots of times during
the startup of zsh. most of them were inside the function `shingetline`.

So This patch add a flag into the function, so that it only call
`winch_(un)?block` witch make the system call only if needed.

I have included a benchmarking test after and before the patch, as you can
see it got rid of 90% of the system calls, and zsh startup and exit times
got reduced by 20%.

Kind regards,

--
Mohamed Elawadi
mohamed@elawadi.net

➜  code git:(master) ✗ /usr/bin/time -p bash -c 'for i in $(seq 1 100); do
zsh -i -c "exit"; done'
real 29.93
user 16.90
sys 7.84

➜  code git:(master) ✗ strace -cf zsh -i -c 'exit'
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 68.12    0.012507           0    169032           rt_sigprocmask
 24.79    0.004552          71        64        28 wait4
  3.13    0.000574           2       271           getdents
------ ----------- ----------- --------- --------- ----------------
100.00    0.018361                180201      1465 total

After applying the patch

➜  code git:(master) ✗ /usr/bin/time -p bash -c 'for i in $(seq 1 100); do
zsh -i -c "exit"; done'
real 25.08
user 14.52
sys 5.49

➜  code git:(master) ✗ strace -cf zsh -i -c 'exit'

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 69.71    0.004623          72        64        28 wait4
  8.40    0.000557           0     16364           rt_sigprocmask
  4.27    0.000283           9        33           clone
  2.86    0.000190           1       271           getdents
------ ----------- ----------- --------- --------- ----------------
100.00    0.006632                 27476      1464 total