util: improve user lookup helper

util: improve user lookup helper

https://bugzilla.redhat.com/show_bug.cgi?id=964358

A future patch needs to look up pw_gid; but it is wasteful
to crawl through getpwuid_r twice for two separate pieces
of information, and annoying to copy that much boilerplate
code for doing the crawl.  The current internal-only
virGetUserEnt is also a rather awkward interface; it's easier
to just design it to let callers request multiple pieces of
data as needed from one traversal.

And while at it, I noticed that virGetXDGDirectory could deref
NULL if the getpwuid_r lookup fails.

* src/util/virutil.c (virGetUserEnt): Alter signature.
(virGetUserDirectory, virGetXDGDirectory, virGetUserName): Adjust
callers.

Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit c1983ba4e3902308054e961fcae75cece73ef4ba)

Conflicts:
	src/util/virutil.c - oom reporting/strdup changes not backported