Fix sign conversion and comparison warnings

Don't ignore fwrite errors

Fixes warning on CentOS 6:

  src/nnn.c:754: warning: ignoring return value of 'fwrite', declared with attribute warn_unused_result

Building on i386 (NetBSD or Debian):

  src/nnn.c: In function 'selectiontofd':
  src/nnn.c:783:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    while (pos <= lastpos) {
               ^
  src/nnn.c:791:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if (pos <= lastpos) {
             ^
  src/nnn.c: In function 'showcplist':
  src/nnn.c:823:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    if (pos && pos == copybufpos)
                   ^
  src/nnn.c: In function 'xlink':
  src/nnn.c:1955:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    while (pos < copybufpos) {

The new location is ~/.config/nnn/.lastd

See #187. The behaviour has always remained so confusing. In my case a 0B file
opens in the GUI editor though I have NNN_USE_EDITOR set. We have a sensible
message in place to help users with what to do next.

Author: @bruce-hill