Fix warnings from clang 4.0.0 and Valgrind 3.12.0

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.)