tool_main: exit at once if out of file descriptors

tool_main: exit at once if out of file descriptors

If the main_checkfds function cannot create new file descriptors in an
attempt to detect of stdin, stdout or stderr are closed.

Also changed the check to use fcntl() to check if the descriptors are
open, which avoids superfluously calling pipe() if they all already are.

Follow-up to facfa19cdd4d0094

Reported-by: Trail of Bits
Closes #9663