sysv-initctl: avoid busy loop without FIFO clients

[pva]

sysv-initctl opened /run/initctl with O_RDONLY | O_NONBLOCK and called read() directly in a loop. Immediately after startup, no client has the FIFO open for writing, so read() returns zero instead of waiting. The daemon treated this as a short request and retried continuously, consuming CPU and flooding syslog with "read: short count".

This can be reproduced on a system using openrc-init by starting the compatibility daemon without connecting an initctl client:

rc-service sysv-initctl start
pid=$(pgrep -x sysv-initctl)
strace -p "$pid" -e read
top -p "$pid"

Before this change, strace repeatedly reports read() returning zero.

Open the read end nonblocking during setup, keep a dummy write end open, and then make the reader blocking. This allows the daemon to sleep in read() while no clients are connected. Mark both descriptors close-on-exec.

[navi-desu]

this is much of the same logic as openrc-init, so i pushed a commit reusing it

also there's no need to repeat the explanation every time, just refer to the old fix

[pva]

@navi-desu Yea, it's a good idea to move code into a helper header. Yet, although the fixes are similar, the triggers are different. In the first case, the issue was triggered by connecting to the socket and sending a signal; here, it is triggered when no listener is present. That's why the descriptions were different.