app.c: make sure that no non-async-signal-safe syscalls are used after

app.c: make sure that no non-async-signal-safe syscalls are used after
fork before exec

Posix does only allow async-signal-safe syscalls after fork before exec.
As asterisk ignores this, functions like TrySystem or System sometimes
end up in a deadlocked child process. The patch prevents the use of
non-async-signal-safe syscalls.

ASTERISK-28776

Change-Id: Idc76365c0592ee3f3b3bd72a4f48f7a098978e8e