mhlsbr.c: Don't strchr(3) non-string NUL-less buffer.

Up to 8 KiB was read into a buffer and then strchr() used to test for a
linefeed.  There was no guarantee the linefeed would be present, else
why test for it, nor that the buffer would contain a NUL to terminate
the search, either from the read bytes, or the bytes not trampled by
read().  Replace the two similar lumps of code with a new
linefeed_typed().  Bug pre-dates git.