Optimize rpmSetCloseOnExec

In case maximum number of open files limit is set too high, both
luaext/Pexec() and lib/doScriptExec() spend way too much time
trying to set FD_CLOEXEC flag for all those file descriptors,
resulting in severe increase of time it takes to execute say
rpm or dnf.

This becomes increasingly noticeable when running with e.g. under
Docker, the reason being:

> $ docker run fedora ulimit -n
> 1048576

One obvious fix is to use procfs to get the actual list of opened fds
and iterate over it. My quick-n-dirty benchmark shows the /proc approach
is about 10x faster than iterating through a list of just 1024 fds,
so it's an improvement even for default ulimit values.

Note that the old method is still used in case /proc is not available.

While at it,

 1. fix the function by making sure we modify (rather than set)
    the existing flags. As the only known flag is FD_CLOEXEC,
    this change is currently purely aesthetical, but in case
    other flags will appear it will become a real bug fix.

 2. get rid of magic number 3; use STDERR_FILENO

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Fixes #444

rpmio/rpmio.c

@@ -1760,18 +1760,43 @@ DIGEST_CTX fdDupDigest(FD_t fd, int id)
     return ctx;
 }
 
+static void set_cloexec(int fd)
+{
+	int flags = fcntl(fd, F_GETFD);
+
+	if (flags == -1 || (flags & FD_CLOEXEC))
+		return;
+
+	fcntl(fd, F_SETFD, flags | FD_CLOEXEC);
+}
+
 void rpmSetCloseOnExec(void)
 {
-	int flag, fdno, open_max;
+	const int min_fd = STDERR_FILENO; /* don't touch stdin/out/err */
+	int fd;
+
+	DIR *dir = opendir("/proc/self/fd");
+	if (dir == NULL) { /* /proc not available */
+		/* iterate over all possible fds, might be slow */
+		int open_max = sysconf(_SC_OPEN_MAX);
+		if (open_max == -1)
+			open_max = 1024;
 
-	open_max = sysconf(_SC_OPEN_MAX);
-	if (open_max == -1) {
-		open_max = 1024;
+		for (fd = min_fd + 1; fd < open_max; fd++)
+			set_cloexec(fd);
+
+		return;
 	}
-	for (fdno = 3; fdno < open_max; fdno++) {
-		flag = fcntl(fdno, F_GETFD);
-		if (flag == -1 || (flag & FD_CLOEXEC))
-			continue;
-		fcntl(fdno, F_SETFD, FD_CLOEXEC);
+
+	/* iterate over fds obtained from /proc */
+	struct dirent *entry;
+	while ((entry = readdir(dir)) != NULL) {
+		fd = atoi(entry->d_name);
+		if (fd > min_fd)
+			set_cloexec(fd);
 	}
+
+	closedir(dir);
+
+	return;
 }