GUACAMOLE-2239: guac_wait_for_fd() does not correctly retry on EINTR.

[bbennett-ks]

Summary

This PR hardens guac_wait_for_fd() against signal interruption (EINTR). This is a subset of GUACAMOLE-2238.

When a signal is delivered (e.g. when a child process exits), some blocking syscalls can return early with -1 with errno = EINTR . This is not a hard failure: the syscall should be retried.

This PR adds EINTR retry loops around blocking calls in various guacamole-server. Equivalent handling for the Windows equivalent was also added.

Without retry, signal (async) delivery can cause guac_wait_for_fd() to intermittently fail.

select Handling

int select(int nfds, fd_set *_Nullable restrict readfds, fd_set *_Nullable restrict writefds, fd_set *_Nullable restrict exceptfds, struct timeval *_Nullable restrict timeout);

Special care has been taken with the select system call, as it has parameters which are modified by the system: how these parameters are handled on retry must be considered. Based on the Linux man page:

On error, -1 is returned, and is set to indicate the error; the file descriptor sets are unmodified, and timeout becomes undefined.

select retry is based on this.

[necouchman]

Also good with this one, but will wait for @mike-jumper comments, as well.

[bbennett-ks]

Moving forward with GUACAMOLE-2238.