BACKPORT: fd_control.c:set_nonblocking() clears too many flags.

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=3546

Reported by @grubba

fd_control.c:set_nonblocking()'s use of fcntl F_SETFL clears too many flags (including the flag O_LARGEFILE), which causes read(2) to fail with EOVERFLOW on files >2GB.

Fixed in Pike 7.5.