Use procfs to detect nr of open fd's if rlimit is too high (bsc#1191324)

[bzeller]

Especially in a VM iterating over all possible fd's to close open ones
right before a exec() slows down zypper unnecessarily. This patch uses
/proc/self/fd to iterate over open fd's in case rlimit is above 1024.

[crrodriguez]

huh...no.. if you want close all open fds you have to use the close_range() system call and only do this /proc/self/fd iteration if invoking syscall return negative value..

[mlandres]

Yes, we could check whether close_range is available. AFAICS not up to 15.3. Don't know if it's in 15.4, but TW seems to have it.

[crrodriguez]

Yes, we could check whether close_range is available. AFAICS not up to 15.3. Don't know if it's in 15.4, but TW seems to have it.

even better, it should just use posix_spawn to execute things if the file action to close file descriptors on exec is available in the C library.. if posix_spawn_file_actions_addclosefrom_np is defined..
It is not that flexible but it is the cheapest/fastest way to run commands . Internally the C library uses it for pretty much all command execution functions (system, popen, wordexp..etc)