Read a 4-byte mtime as unsigned (old-protocol).

Read a 4-byte mtime as unsigned (old-protocol).

When conversing with a protocol 29 or earlier rsync, the modtime values
are arriving as 4-byte integers.  This change interprets these short
values as unsigned integers, allowing the time that can be conveyed to
range from 1-Jan-1970 to 7-Feb-2106 instead of the signed range of
13-Dec-1901 to 19-Jan-2038.  Given that we are fast approaching 2038,
any old-protocol transfers will be better served using the unsigned
range rather than the signed.

It is important to keep in mind that protocol 30 & 31 convey the full
8-byte mtime value (plus nanoseconds), allowing for a huge span of time
that is not affected by this change.