fix github pull request #26 - validating #explore

Fixes #26

Report stated that the attempt to look up the player's username
(on Unix) failed (reason unknown) and nethack refused to allow the
player to execute the #explore command even though sysconf was set
to use character names (CHECK_PLNAME=1) instead of user names.
Setting EXPLORERS to "*" overcomes this glitch, but the fix moves
a bit of code around to honor CHECK_PLNAME before fetching username
so that that isn't necessary.

I ended up doing some formattng clean up (replace tabs with spaces,
whitespace cleanup in 'port_insert_pastebuf()').  The actual change
to fix #26 is only a few lines.

doc/fixes36.3

@@ -1,4 +1,4 @@
-$NHDT-Branch: NetHack-3.6 $:$NHDT-Revision: 1.125 $ $NHDT-Date: 1570318925 2019/10/05 23:42:05 $
+$NHDT-Branch: NetHack-3.6 $:$NHDT-Revision: 1.126 $ $NHDT-Date: 1570408209 2019/10/07 00:30:09 $
 
 This fixes36.3 file is here to capture information about updates in the 3.6.x
 lineage following the release of 3.6.2 in May 2019. Please note, however,
@@ -209,6 +209,8 @@ tty: revert the attempt to fix "message line anomaly: if autodecribe feedback
 	wrapped to second line, the wrapped portion wasn't erased when a
 	shorter line was shown or getpos was dismissed" because it disrupted
 	prompts that spanned more than one line, a more significant issue
+unix: sysconf CHECK_PLNAME=1 wouldn't work if attempt to obtain unix username
+	failed even though it didn't need that username
 
 
 Platform- and/or Interface-Specific Fixes or Features