evas/cserve2: Fix interrupt on select in edje_cc

evas/cserve2: Fix interrupt on select in edje_cc

Summary:
Block SIGCHLD during select().
This fixes a bug with edje_cc when EVAS_CSERVE2=1: Fixes T464.

select() used to return prematurately with EINTR because the
app received some unexpected signals. In particular SIGCHLD
is received when a child terminates, but this is not a reason
to cancel the image load.

In theory, all blocked signals in pselect() should be pending
until pselect returns, so any SIGCHLD should still trigger
the app's signal handler.

Reviewers: cedric

CC: raster, cedric
Maniphest Tasks: T464

Differential Revision: https://phab.enlightenment.org/D357