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