Fix most InnoDB and XtraDB warnings from clang 4.0.

In functions that declare pointer parameters as nonnull,
remove nullness checks, because GCC would optimize them away anyway.

Use #ifdef instead of #if when checking for a configuration flag.

Clang says that left shifts of negative values are undefined.
So, use ~0U instead of ~0 in a number of macros.

Some functions that were defined as UNIV_INLINE were declared as
UNIV_INTERN. Consistently use the same type of linkage.

ibuf_merge_or_delete_for_page() could pass bitmap_page=NULL to
buf_page_print(), conflicting with the _attribute_((nonnull)).

Fix some bogus-looking Valgrind warnings.

buf_block_init(): Initialize buf_page_t::flush_type.
For some reason, Valgrind 3.12.0 would seem to flag some
bits in adjacent bitfields as uninitialized, even though only
the two bits of flush_type were left uninitialized. Initialize
the field to get rid of many warnings.

mach_write_to_1(), mach_write_to_2(), mach_write_to_3():
Rewrite the assertions that ensure that the most significant
bits are zero. Apparently, clang 4.0 would optimize expressions
of the form ((n | 0xFF) <= 0x100) to (n <= 0x100). The redundant
0xFF was added in the first place in order to suppress a
Valgrind warning. (Valgrind would warn about comparing uninitialized
values even in the case when the uninitialized bits do not affect
the result of the comparison.)