If char is unsigned (as on ARM) subtracting a larger number would
result in a wrap around, not a negative value.

  src/nnn.c: In function 'readpipe':
  src/nnn.c:4325:11: warning: comparison is always false due to limited
  range of data type [-Wtype-limits]

     if (ctx < 0 || ctx > CTX_MAX)
             ^

Fixes the build on NetBSD 9 on arm64.

GCC expands alloca() to __builtin_alloca() but only in nonstandard mode,
e.g. when -std=... is not supplied.  In standards mode (with -std=...)
alloca() is left undefined by GCC. The C library may define it but it
also may not, as on NetBSD on arm64:

    $ uname -srp
    NetBSD 9.0 aarch64

    $ cat alloca.c
    #include <stdlib.h>
    int main() { char *p = alloca(10); }

    $ gcc alloca.c

    $ gcc -std=c99 alloca.c
    alloca.c:(.text+0xc): warning: Warning: reference to the libc
     supplied alloca(3); this most likely will not work. Please use the
     compiler provided version of alloca(3), by supplying the
     appropriate compiler flags (e.g. not -std=c89).
    ld: alloca.c:(.text+0xc): undefined reference to `alloca'

The fix is to either not use standards mode (undesirable) or to
explicitly use the builtin, which is what this patch does.

This is also sufficient for Solarius/Illumos so that check and include
are removed.

Fixes #656

Selection is cleared after successful:
- file removal
- batch rename
- link creation

However, selection is not cleared after:
- archive creation: user may want to delete the files after archiving

We leave it undefined on Solaris/Illumos because no such options exist,
so only use it if we have it.

Note that this is not compatible with 4 contexts.

This supports using a power of 2 number of contexts like 2, 4 or 8.
Note that the number of contexts are not backward compatible. So if
user is having 8 contexts and plans to use 4, his sessions with > 1
contexts open may break.

We can have a make option to enable 8 contexts. 4 remains default.

This reverts commit 2232772db4.

This allow using both NNN_OPTS and options with arguments from argv.

It's a little annoying that one has to press ^L followed by Esc to go back
to the normal view. We cannot reload the directory on Esc because that will
break the filter filtered view feature. So we take this route.

* Add -a option to generate a temporary NNN_FIFO

* Add documentation for -a option

* plugins/README.md: promote the use of -a

This obsoletes the global FIFO unlink trick, so I remove mentions of it.
@jarun update:

Polish -a