rename to interposition

refs #144
we now intercept all mem alloc functions without segfaults

for now, i separated the memory preload functions into its own
file. that will eventually merge back in with preload.c after
moving all funcs  to the cleaner preload design as part of #37.

there are still a number of pointers that we are not tracking
or tracking incorrectly. more analysis needed there.

refs #37, refs #144
refactored preload functions

system calls and all shadow functions are now searched
at library load time with dlsym. openssl (and other dynamically
loaded functions) must still be lazily loaded to ensure the symbol
is resolved by the time we search for it.

not sure if it ended up simpler than the previous method,
but its much easier to understand now, i think.

closes #37
no longer install the nonexistant preload.h

refs #37
thread-safe malloc interception

preloading is now done properly under both single and
multi-threaded execution.

refs #144