wg-quick: kill route monitor when loop terminates

wg-quick: kill route monitor when loop terminates

If the route monitor doesn't attempt to write more to stdout, then this
leaves a process hanging around. Kill it explicitly. We also switch to
using exec in the process substitution, to reduce a bash process.

Closes: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=255286
Reported-by: Christos Chatzaras <chris@cretaforce.gr>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>