lib: Make error.c more like error(3)

lib: Make error.c more like error(3)

Fix some issues with the error reimplementation to make it match
the specification for error(3).

Flush stdout before printing to stderr.  Also flush stderr afterwards,
which is not specified in the man page for error(3), but is what
bionic does.

error(3) prints strerror(errnum) if and only if errnum is nonzero,
but verr prints strerror(errno) unconditionaly.  When errnum is nonzero
copy it to errno and use verr, and when it is not set use verrx that
doesn't print errno.

error(3) only exits if status is nonzero, but verr exits uncondtionally.
Use vwarn/vwarnx when status is zero, which don't exit.

Signed-off-by: Colin Cross <ccross@google.com>