This reverts commit 37e5e6189a.

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.