Name the implementation of brk and sbrk sys_break().

The break() system call was renamed (several times) starting in v3
AT&T UNIX when C was invented and break was a language keyword. The
last vestage of a need for it to be called something else (eg obreak)
was removed in r225617 which consistantly prefixed all syscall
implementations.

Reviewed by: emaste, kib (older version)
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D15638