gh-112970: Detect and use closefrom() when available (#112969)

glibc-2.34 implements closefrom(3) using the same semantics as on BSD.
Check for closefrom() in configure and use the check result in
fileutils.c, rather than hardcoding a FreeBSD check.

Some implementations of closefrom() return an int. Explicitly discard 
the return value by casting it to void, to avoid future compiler
warnings.

Signed-off-by: Sam James <sam@gentoo.org>