Avoid the creation of too many strings in 'package'

Both when setting a path and searching for a file ('searchpath'),
this commit reduces the number of intermediate strings created
in Lua.
(For setting a path the change is not relevant, because this is
done only twice when loading the module. Anyway, it is a nice example
of how to use auxlib buffers to manipulate strings in the C API.)

lgc.h

@@ -127,7 +127,7 @@
 
 /*
 ** some gc parameters are stored divided by 4 to allow a maximum value
-** larger than 1000 in a 'lu_byte'.
+** up to 1023 in a 'lu_byte'.
 */
 #define getgcparam(p)	((p) * 4)
 #define setgcparam(p,v)	((p) = (v) / 4)