Compat: cleanup several config.h hacks

Compat: cleanup several config.h hacks

* removes the xmemcpy and xmemmove hacks. Squid has been building for some
  long time without them being consistently used all-over. No complaints.
  bcopy() alternative is still use if needed. However main code can now
  use memcpy( )and memmove() without special X knowledge.

* shuffle strnstr replacement into libcompat from libmisc

* shuffles xis*() function wrappers for is*() with type-casting into compat

No operational changes.