Add support for FreeBSD CURRENT (aka freebsd13)

Using the last FreeBSD-CURRENT (development snapshot) the libc build, but tests fail.
This patch detects and supports FreeBSD CURRENT as freebsd13, and reworks the conditional compilation to use the `freebsd11` attribute instead of `not(freebsd12)`
For now, freebsd13 is reusing all freebsd12 definitions, except for `ELAST`
While here, add a new `errno`introduced in freebsd13