InnoDB contains a number of parsing functions that take the start and
end pointer to the a buffer, and return the pointer to the next byte
in the buffer (or NULL in case the buffer was incomplete). The parsed
output is returned in an output parameter.

Compilers may issue bogus warnings about the output being
uninitialized.  It would be more natural programming style to return
the parsed output, and update the pointer to the start of the buffer
(make it an in,out parameter).

mach_read_compressed(): Replace with mach_read_next_compressed().

mach_ull_read_compressed(), mach_ull_get_compressed_size():
Replace with mach_u64_read_next_compressed().

mach_ull_get_much_compressed_size(): Replace with
mach_read_next_much_compressed().

mach_u64_read_much_compressed(): Renamed from
mach_ull_read_much_compressed(). One caller remains.

mach_get_compressed_size(): Replace with
mach_read_next_compressed().

mach_parse_compressed(), mach_u64_parse_compressed(): Return the
parsed output. Make the first parameter in,out.

Various functions: Add const qualifiers to the ptr, end_ptr.
On return, cast away the constness for now. We should introduce const
qualifiers to everywhere where we are parsing read-only buffers, such
as undo log and redo log.

rb#4569 approved by Vasil Dimov