MFC r221012: sh: Check setuid()/setgid() return values.

MFC r221012: sh: Check setuid()/setgid() return values.

If the -p option is turned off, privileges from a setuid or setgid binary
are dropped. Make sure to check if this succeeds. If it fails, this is an
error which will cause the shell to abort except in interactive mode or if
'command' was used to make 'set' or an outer 'eval' or '.' non-special.

Note that taking advantage of this feature and writing setuid shell scripts
seems unwise.

git-svn-id: svn://svn.freebsd.org/base/stable/8@221515 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f