libc: add clearenv function

libc: add clearenv function

The clearenv(3) function allows us to clear all environment
variable in one shot. This may be useful for security programs that
want to control the environment or what variables are passed to new
spawned programs.

Reviewed by:	scf, markj (secteam), 0mp (manpages)
Differential Revision:	https://reviews.freebsd.org/D28223