Rename nitems and string variables to avoid collisions

Rename nitems and string variables to avoid collisions

Rename the `nitems` variable to `num_items` to avoid collisions with the
macro in sys/param.h for counting elements in an array

Similarly, rename `string` to `string_arr` to avoid future collisions with
potential keywords, as well as make it clear that `string_arr` isn't a char*
value, but instead a char** value.

Differential Revision: https://reviews.freebsd.org/D4769 (part of larger diff)
MFC after: 5 days
Reviewed by: oshogbo
Sponsored by: EMC / Isilon Storage Division