o Integrate security enhancements from OpenBSD.

o Integrate security enhancements from OpenBSD.
  - Don't assume environment variable HOME is not NULL.
o Integrate standards compliance from NetBSD.
  - Allow -- before the command.
  - Blocking SIGQUIT isn't standards compliant.
  - Proper exit(3) levels.
  - Actually append to nohup.out (as documented and required
    by standard) instead of clobbering it.
o Remove some FreeBSD specific access(2) cruft (relating to
  incorrect appending).
o Document the fact that two or more instances of nohup can
  append to the same file.
o Constify; Staticize functions; Set WARNS?=2

Reviewed by:	bde
Approved by:	des
Obtained from:	NetBSD, OpenBSD
MFC after:	9 days