get_rnd_text() for rumors and other stuff

Solve the uneven distribution situation that has been present for
picking random rumors for a long time and for random engravings,
epitaphs, and hallucinatory monster names since 3.6.0.  This relies
on the previous partial solution where short lines have been padded
to a longer length.  When that length is N and random seek lands in
a long line of length L, retry if the position is in the first L-N
characters.  Put differently, it if takes more than N characters to
reach the next newline, reject that random seek and try again.  This
effectively makes long lines behave as if they had the same length
of N as the short lines have been padded to and when all lines are
the same length, all entries have the same chance to be chosen.

doc/fixes37.0

@@ -701,6 +701,8 @@ selection of random engravings, epitaphs, and hallucinatory monster names had
 	ones which follow shorter than average lines are least likely; use
 	same workaround as for rumors:  pad the shortest lines; result isn't
 	uniforn distribution but is better (tradeoff vs size; see makedefs)
+make selection of random rumors, engravings, epitaphs, and hallucinatory monst
+	names have uniform distribution by handling long lines specially
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository