MFC r347892:
LinuxKPI: Finalize import of seq_file.

seq_file.h and linux_seq_file.c was imported form ports earlier but
linux_seq_file.c was never compiled in with the module. With this
commit base seq_file will replace ports seq_file and it required a
few modifications to not break functionality and build.

Reviewed by: hps
Approved by: imp (mentor), hps