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