unistd: drop argument names from setpgrp(3) prototype.

They are useless and don't match the style of the header.
While here adjust the comment with tabs.

Suggested by:	kevinlo


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271878 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f