On Microsoft Windows, ReadFile() as well as WriteFile() limit the size
of the request to DWORD, which is 32 bits (at most 4 GiB - 1) also on
64-bit systems.

On FreeBSD, sysctl debug.iosize_max_clamp could limit the size of a
write request to INT_MAX. The size of a read request is always limited
to INT_MAX. This would allow the request size to be 4095 bytes more than
the Linux limit (0x7ffff000 according to "man 2 read" and "man 2 write").

On OpenBSD, Solaris and possibly NetBSD, the read request size is limited
to SSIZE_T_MAX, which would be half the current maximum
innodb_log_buffer_size. This should be not much of an issue anyway,
because on contemporary 64-bit platforms, the virtual addresses are
limited to 48 bits.

IBM AIX documentation mentions OFF_MAX which would apply when
a 64-bit application is running on a 32-bit kernel.

Let us declare innodb_log_buffer_size as 32-bit unsigned and make the
maximum 0x7ffff000, to be compatible with the least common
denominator (Linux).

The maximum innodb_sort_buffer_size already was 64 MiB,
which is not a problem.

SyncFileIO::execute(): Assert that the size of a synchronous read or
write request is limited to the maximum.

Reviewed by: Vladislav Vaintroub