seekdir_basic: fix various Coverity issues

Address..

• .. resource leaks of file descriptors and memory
• .. unchecked return values from creat(2), mkdir(2), and telldir(3)
• .. potential NULL derefs after calling readdir(3)

MFC after: 1 week
Reported by: Coverity
CID: 975255, 975256, 976989, 978989, 978990