vircommand: Make sysconf(_SC_OPEN_MAX) failure non-fatal

vircommand: Make sysconf(_SC_OPEN_MAX) failure non-fatal

The point of calling sysconf(_SC_OPEN_MAX) is to allocate big
enough bitmap so that subsequent call to
virCommandMassCloseGetFDsDir() can just set the bit instead of
expanding memory (this code runs in a forked off child and thus
using async-signal-unsafe functions like malloc() is a bit
tricky).

But on some systems the limit for opened FDs is virtually
non-existent (typically macOS Ventura started reporting EINVAL).

But with both glibc and musl using malloc() after fork() is safe.
And with sufficiently new glib too, as it's using malloc() with
newer releases instead of their own allocator.

Therefore, pick a sufficiently large value (glibc falls back to
256, [1], Darwin to 10240 [2] so 10240 should be good enough) to
fall back to and make the error non-fatal.

1: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/getdtsz.c;h=4c5a6208067d2f9eaaac6dba652702fb4af9b7e3;hb=HEAD
2  https://github.com/apple/darwin-xnu/blob/main/bsd/sys/syslimits.h#L104

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>