Move inner loop logic out of sysctl_sysctl_next_ls().

Move inner loop logic out of sysctl_sysctl_next_ls().

Refactor sysctl_sysctl_next_ls():
* Move huge inner loop out of sysctl_sysctl_next_ls() into a separate
 non-recursive function, returning the next step to be taken.
* Update resulting node oid parts only on successful lookup
* Make sysctl_sysctl_next_ls() return boolean success/failure instead of errno,
 slightly simplifying logic

Reviewed by:	freqlabs
Differential Revision:	https://reviews.freebsd.org/D27029