tailf: clean up gcc warnings & fix use of errno

tailf: clean up gcc warnings & fix use of errno

Fix strict gcc warnings in tailf that come from using:
  ("-Wall -Wp,-D_FORTIFY_SOURCE=2")

tailf.c:111: warning: ignoring return value of 'fwrite', declared with attribute warn_unused_result

Also, tailf uses perror() for error reporting, but it inserts
an fprintf call first, so perror() is actually reporting the
result of the fprintf() call, not the failing call; change
the code to print the message by using strerror() instead.

Builds cleanly on x86_32 and x86_64.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Karel Zak <kzak@redhat.com>