Add CHECK_PLNAME to sysconf

Setting CHECK_PLNAME to 1 makes WIZARDS, EXPLORERS, and SHELLERS
check the player name instead of the user's login name.

This is mostly useful for public servers which have external
login system and don't create user accounts for players.

doc/Guidebook.tex

@@ -3905,6 +3905,10 @@ A string explaining how to recover a game on this system (no default value).
 0 or 1 to disable or enable, respectively, the SEDUCE option (see the source)
 for details on this function.
 %.lp
+\item[\ib{CHECK\verb+_+PLNAME}]
+Setting this to 1 will make the EXPLORERS, WIZARDS, and SHELLERS check
+for the player name instead of the user's login name.
+%.lp
 \item[\ib{CHECK\verb+_+SAVE\verb+_+UID}]
 0 or 1 to disable or enable, respectively, the UID checking for savefiles.
 \elist