Use #include "..." for our own headers, <...> for others

Use #include "..." for our own headers, <...> for others

System headers should be included with <...>, our own headers with
"...".  Offenders tracked down with an ugly, brittle and probably
buggy Perl script.  Previous iteration was commit a9c94277f0.

Delete inclusions of "string.h" and "strings.h" instead of fixing them
to <string.h> and <strings.h>, because we always include these via
osdep.h.

Put the cleaned up system header includes first.

While there, separate #include from file comment with exactly one
blank line.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180201111846.21846-2-armbru@redhat.com>