Linux (including Android), MacOS and BSD all have particular rules
conditions Separate them out into three sections to make the logic for
each clear.

Change of strategy allows users to specify '-c' twice to erase the
scrollback buffer.

Guard against absurd limits; on MacOS 11.1 may be 2**63-1.

Co-authored-by: Philipp Metzner <philipp.metzner@reactive-robotics.com>

To allow Ctrl-C to function

Some versions of Android do not allow unprivileged access to
/proc/sys/fs/inotify/max_user_watches, in this case we can still use the
original method rather than fail.

README: fix examples of using find(1)

In commit eadd72f11e, the examples of using ls(1) were changed to use
find(1).  Even though the examples might seam to work, the correct way
to search for file names is to use the `-name' command and for files in
general, the `type f' command.

Typically, we should change as well the very first example, which
returns *both* source files *and* directories.  The comment suggests we
are interested only in source files.

In GNU find(1) we could omit the search directory, by default is current
working directory, but this is not portable in POSIX or BSD find's.

Makefile: include "regress" in PHONY targets

printf() is not signal-safe, but we're bailing out immediately

busybox nc(1) does not support domain sockets

ls(1) is not consistent in the way it formats output for other commands to process. find(1) is a more standard way to give input to another command.

When trying to build with gcc 10, there is a linker error:

    /usr/bin/ld: /tmp/ccjRQbpX.o:/builddir/build/BUILD/entr-4.4/data.h:33: multiple definition of `files'; /tmp/ccHpowMW.o:/builddir/build/BUILD/entr-4.4/missing/../data.h:33: first defined here
    collect2: error: ld returned 1 exit status

This is a known thing in gcc 10:

https://gcc.gnu.org/gcc-10/porting_to.html#common

This patch adds extern keyword to the header file and removes it from
exactly one source file.

Thanks to suggestions from Stephen Gregoratto <dev@sgregoratto.me>

This was to indicate that the return value of a function is discarded.
It serves no functional purpose, and was never used consistently.

Limited by /proc/sys/fs/inotify/max_user_watches

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