Added -t option to display field types on sysctl(8).

[araujo]

Summary

PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=203918

Added -t option to display field types. It may be used together with -d option.
This made easier to find field types when toying sysctls in systat.

Test Plan

DOCS: igor -R and mandoc -Tlint

sysctl(8):
sysctl -a -t
sysctl -dt security.jail.mount_allowed
sysctl -t security.jail.mount_allowed

[cem]

Looks good to me modulo minor nitpicks inline.

sbin/sysctl/sysctl.c

915 ↗

(On Diff #10950)

Use typename != NULL (for non-boolean types) rather than implicitly converting a pointer to bool.

917–919 ↗

(On Diff #10950)

Drop the comments, they're not helpful here.

[araujo]

Address @cem comments.
Please, @cem could you double review the changes with the typename?

Best,

[araujo]

Fix an identation problem. My bad!

[cem]

Looks good to me, thanks!