ttys.5: Improve manpage markups and update the examples

Reviewed-by: swildner

ttys: Add 'ifexists' option to enable ttys only if exists

Implement the 'ifexists' ttys option in init(8) to enable a tty only if
it exists. This allows one to turn off getty for ttys that aren't
p

ttys: Add 'ifexists' option to enable ttys only if exists

Implement the 'ifexists' ttys option in init(8) to enable a tty only if
it exists. This allows one to turn off getty for ttys that aren't
present (e.g., on a headless system), and thus prevent getty error logs
from filling up /var/log/messages; e.g.,

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Sep 20 09:13:07 microserver getty[236362]: open /dev/ttyv0: No such file or directory
Sep 20 09:13:07 microserver getty[236404]: open /dev/ttyv0: No such file or directory
Sep 20 09:13:07 microserver getty[236405]: open /dev/ttyv0: No such file or directory
Sep 20 09:13:07 microserver getty[236406]: open /dev/ttyv0: No such file or directory
Sep 20 09:13:07 microserver getty[236407]: open /dev/ttyv0: No such file or directory
Sep 20 09:13:07 microserver init: getty repeating too quickly on port /dev/ttyv0, sleeping 30 secs
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Update the ttys(5) and getttyent(3) man pages accordingly.

The updates to ttys files will follow.

Discussed-with: dillon
Credit: https://reviews.freebsd.org/D10037

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