Reimpement read_file[_of]_lines to be much faster

Instead of reading char-by-char, read whole file or read stream by
pieces of 4KiB each (for non-seekable streams, e.g. pipes).

Thanks to Robert Sarkozi.
Better check for empty file in load_view_data()
Document better performance of less-like view
Merge branch 'increase-speed-of-view-mode-data-load'

Optimize reading list of lines from file.  Do this as one operation,
when possible.

Thanks to Robert Sarkozi.