Allow Dir.home to work for non-login procs when $HOME not set

	Allow the 'Dir.home' method to reliably locate the user's home directory when
	all three of the following are true at the same time:

	1. Ruby is running on a Unix-like OS
	2. The $HOME environment variable is not set
	3. The process is not a descendant of login(1) (or a work-alike)

	The prior behavior was that the lookup could only work for login-descended
	processes.

	This is accomplished by looking up the user's record in the password database
	by uid (getpwuid_r(3)) as a fallback to the lookup by name (getpwname_r(3))
	which is still attempted first (based on the name, if any, returned by
	getlogin_r(3)).

	If getlogin_r(3), getpwnam_r(3), and/or getpwuid_r(3) is not available at
	compile time, will fallback on using their respective non-*_r() variants:
	getlogin(3), getpwnam(3), and/or getpwuid(3).

	The rationale for attempting to do the lookup by name prior to doing it by uid
	is to accommodate the possibility of multiple login names (each with its own
	record in the password database, so each with a potentially different home
	directory) being mapped to the same uid (as is explicitly allowed for by
	POSIX; see getlogin(3posix)).

	Preserves the existing behavior for login-descended processes, and adds the
	new capability of having Dir.home being able to find the user's home directory
	for non-login-descended processes.

	Fixes [Bug #16787]

	Related discussion:
	https://bugs.ruby-lang.org/issues/16787
	https://github.com/ruby/ruby/pull/3034


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67931 b2dd03c8-39d4-4d8f-98ff-823fe69b080e