feat: attempt to autodetect need for -N switch to nc

autoload/clipper/private.vim

@@ -10,15 +10,44 @@ endfunction
 function! clipper#private#clip() abort
   if exists('s:invocation') && s:invocation != ''
     call system(s:invocation, @0)
-  elseif executable('nc') == 1
+  elseif clipper#private#executable() != ''
+    let l:executable = clipper#private#executable()
     let l:address = get(g:, 'ClipperAddress', 'localhost')
     let l:port = +(get(g:, 'ClipperPort', 8377)) " Co-erce to number.
     if l:port
-      call system('nc ' . l:address . ' ' . l:port, @0)
+      " nc
+      call system(l:executable . ' ' . l:address . ' ' . l:port, @0)
     else
-      call system('nc -U ' . l:address, @0)
+      " nc -U
+      call system(l:executable . ' -U ' . l:address, @0)
     endif
   else
     echoerr 'Clipper: nc executable does not exist'
   endif
 endfunction
+
+let s:executable=''
+
+function! clipper#private#executable() abort
+  if s:executable == '' && executable('nc') == 1
+    " Try to figure out whether -N switch is supported and required.
+    " Examples:
+    " - Ubuntu, FreeBSD and similar:
+    "     Supports and requires -N:
+    "     "shutdown(2) the network socket after EOF on the input"
+    "     http://manpages.ubuntu.com/manpages/bionic/man1/nc_openbsd.1.html
+    "     https://www.freebsd.org/cgi/man.cgi?nc
+    " - Darwin:
+    "     Supports but does not require -N; it does something else:
+    "     "Number of probes to send before generating a write timeout event"
+    " - CentOS etc:
+    "     Does not support or need -N.
+    let l:help = system('nc -h')
+    if match(l:help, '\v\s-N\s.+shutdown>') != -1
+      let s:executable = 'nc -N'
+    else
+      let s:executable = 'nc'
+    endif
+  endif
+  return s:executable
+endfunction