Truncate strings in tarCreateHeader() with strlcpy(), not sprintf().

This supplements the GNU libc bug #6530 workarounds introduced in commit
54cd4f04.  On affected systems, a
tar-format pg_basebackup failed when some filename beneath the data
directory was not valid character data in the postmaster/walsender
locale.  Back-patch to 9.1, where pg_basebackup was introduced.  Extant,
bug-prone conversion specifications receive only ASCII bytes or involve
low-importance messages.