split: switch to getline() for line/pattern matching

Get rid of split's home-grown logic for growing the buffer; arbitrarily
breaking at LONG_MAX bytes instead of 65536 bytes gives us much more
wiggle room. Additionally, we'll actually fail out entirely if we can't
fit a line, which makes noticing this class of problem much easier.

Reviewed by: bapt, emaste, pauamma
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D36323