Add bounds checking to the setenv part of the kernel environment.

Add bounds checking to the setenv part of the kernel environment.

This has no security implications since only root is allowed to use
kenv(1) (and corrupt the kernel memory after adding too much variables
previous to this commit).

This is based upon the PR [1] mentioned below, but extended to check both
bounds (in case of an overflow of the counting variable) and to comply
to the style of the function. An overflow of the counting variable
shouldn't happen after adding the check for the upper bound, but better
safe than sorry (in case some other function in the kernel overwrites
random memory).

An interested soul may want to add a printf to notify root in case the
bounds are hit.

Also allocate KENV_SIZE+1 entries (the array is NULL-terminated), since
the comment for KENV_SIZE says it's the maximum number of environment
strings. [2]

PR:		83687 [1]
Submitted by:	Harry Coin <harrycoin@qconline.com> [1]
Submitted by:	Ariff Abdullah <skywizard@MyBSD.org.my> [2]