Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

top(1): remove special handling of load > 5

When the load is "high" (an arbitrary value) top(1) previously moved the
cursor to the top-left of the screen as an acknowledgment. In practice,
on modern machines, even relatively slow ones, it looked more like a
glitch. Remove the logic.

top(1): cleanup memory allocation and warnings

- Prefer calloc over malloc. This is more predicable and we're not in a
performance sensitive context. [1]
- Remove bogus comment (obsolete from prior commit). [2]
- Remove void casts and type casts of NULL
- Remove redundant declaration of 'quit'
- Add additional const

Reported by: kib [1], vangyzen [2]

top(1): remove wrapper around putchar

This appears to have been written for portability which we no longer
need.

top(1): clean much of WARNS=3 issues

There is still one glaring issue: new_message is not a protoype, but
can't be trivially converted since it uses K&R style var-args.

top(1): Migrate top to usr.bin

We've been maintaining top(1) for a long time, and the upstream
hasn't existed/been used in similarly as long. Make it clear that we own
top(1)

Tested with 'make universe'. Everything passed except MIPS which failed
for unrelated reasons. Install also tested for amd64.

Reviewed by: sbruno
No objections: imp, mmacy
Differential Revision: https://reviews.freebsd.org/D15387