We initially thought it was a safe bet that opal_gethostname() would
never be called before opal_init(). However, it turns out that there
are some cases -- e.g., developer debugging -- where it is useful to
call opal_output() (which calls opal_gethostname()) before
opal_init().
Hence, we need to guarantee that opal_gethostname() always returns a
valid value. If opal_gethostname() finds NULL in
opal_process_info.nodename, simply call the internal function to
initialize opal_process_info.nodename.
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>