Commit message:
```
Discourage the use of sysexits(3) in new code
This commit removes an incorrect statement from the sysexits(3) manual page
about the documented exit values being encouraged by style(9). This has not
been true since 2008 (r186224).
In addition, a new section is added at the top of the manual to discourage
developers from using sysexits(3) in new code. The usual problems people
have with sysexits(3) is that they are not portable, it's hard to pick
a good value in most situations and that good error messages are probably
better anyway.
Obtained from: OpenBSD (partially)
```
Related links:
- https://reviews.freebsd.org/D24224
- https://reviews.freebsd.org/rS186224