redo system compat headers

 * define u_int8_t and u_int32_t for solaris
 * add machine/endian.h compatibility, removing leading _ variants
 * work around the string/strings Posix SUS confusion on solaris
 * always #include_next at the front of the header
 * use LIBCRYPTOCOMPAT_ prefix to avoid conflicts
prep for building getentropy on osx/solaris

only build getentropy as-needed, per-platform
add overrides for system arc4random implementations
add PLATFORM_LDFLAGS for systems that need special flags
update which files we build
update tests for out-of-tree builds

also added arc4random fork test driver to run all available permutations
update ignore file