mfiutil: Handle truncation in mfi_next_learn_time

Only trim the trailing newline if the output from ctime() was not
truncated. To simplify the code, use strlcpy() instead of snprintf()
since strlcpy() can't fail with a negative size (and the previous code
probably didn't handle those errors properly given the int -> size_t
conversion).

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D50882