[krbdev.mit.edu #8230] Improper _findfirst() usage can cause crashes on 64-bit Windows

[Greg Hudson via RT]

_findfirst() has a return type of intptr_t.  Code which assigns its 
value to a "long" variable, and then passes the result to _findnext(), 
can crash on 64-bit Windows where long is 32 bits and pointers are 64 
bits.

This bug can result in build failures running libecho as follows 
(reported by github user egonk):

NMAKE : fatal error U1077: '..\..\util\windows\obj\AMD64\rel\libecho' : 
return code '0xc0000005'

This misuse is also present in the opendir()/readdir()/closedir() shims 
in util/support/plugins.c.