patch 8.1.0914: code related to findfile() is spread out

Problem:    Code related to findfile() is spread out.
Solution:   Put findfile() related code into a new source file. (Yegappan
            Lakshmanan, closes #3934)
diff --git a/src/Make_bc5.mak b/src/Make_bc5.mak
index b2977e7..eb38dd7 100644
--- a/src/Make_bc5.mak
+++ b/src/Make_bc5.mak
@@ -544,6 +544,7 @@
 	$(OBJDIR)\ex_getln.obj \
 	$(OBJDIR)\farsi.obj \
 	$(OBJDIR)\fileio.obj \
+	$(OBJDIR)\findfile.obj \
 	$(OBJDIR)\fold.obj \
 	$(OBJDIR)\getchar.obj \
 	$(OBJDIR)\hardcopy.obj \