res: add nmsg_res_errno
output_nmsg: return nmsg_res_errno when an underlying write() fails
nmsgtool: make debug level 1 the default debug level
nmsg: add nmsg_get_debug() to retrieve the value of _nmsg_global_debug
dnsqr: check nmsg_get_debug() before printing to stderr
nmsg/: ensure that all debugging prints check for a non-zero debugging level
nmsgtool: make debug level 1 the default debug level
nmsg/: use an _nmsg_dprintf() macro instead of checking _nmsg_global_debug and fprintf()ing directly
io: use an _nmsg_dprintfv() macro for debugging prints