lib: use fsync() for config writes, plug fd leak

lib: use fsync() for config writes, plug fd leak

sync() has a HUGE impact on systems that perform actual I/O, i.e. real
servers...

Also, we were leaking a fd on each config write ever since
c5e69a0 "lib/vty: add separate output fd support to VTYs"
(by myself :( ...)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>