err: Add err(), warn(), errx() and warnx() familiy of functions

Some systems such as Windows or musl-libc based ones do not have these
BSD extensions. In addition libbsd itself is making use of the warnx()
functions, so we better provide these interfaces in case they are
missing.
1 job for master in 1 minute and 11 seconds (queued for 20 seconds)
Status Job ID Name Coverage
  Test
passed #484830
test

00:01:11