kern.tty_info_kstacks: add a compact format

Add a more compact display format for kern.tty_info_kstacks inspired by
procstat -kk. Set it as a default one.

sysctl kern.tty_info_kstacks=1

kern.tty_info_kstacks: 0 -> 1

sleep 2

^T
load: 0.17 cmd: sleep 623 [nanslp] 0.72r 0.00u 0.00s 0% 2124k
#0 0xffffffff80c4443e at mi_switch+0xbe
#1 0xffffffff80c98044 at sleepq_catch_signals+0x494
#2 0xffffffff80c982c2 at sleepq_timedwait_sig+0x12
#3 0xffffffff80c43af3 at _sleep+0x193
#4 0xffffffff80c50e31 at kern_clock_nanosleep+0x1a1
#5 0xffffffff80c5119b at sys_nanosleep+0x3b
#6 0xffffffff810ffc69 at amd64_syscall+0x119
#7 0xffffffff810d5520 at fast_syscall_common+0x101
sleep: about 1 second(s) left out of the original 2
^C

sysctl kern.tty_info_kstacks=2

kern.tty_info_kstacks: 1 -> 2

sleep 2

^T
load: 0.24 cmd: sleep 625 [nanslp] 0.81r 0.00u 0.00s 0% 2124k
mi_switch+0xbe sleepq_catch_signals+0x494 sleepq_timedwait_sig+0x12
sleep+0x193 kern_clock_nanosleep+0x1a1 sys_nanosleep+0x3b
amd64_syscall+0x119 fast_syscall_common+0x101
sleep: about 1 second(s) left out of the original 2
^C

Suggested by: avg
Reviewed by: mjg
Relnotes: yes
Sponsored by: Mysterious Code Ltd.
Differential Revision: https://reviews.freebsd.org/D25487