o QsNet: Use /etc/elanhosts for translating hostnames to ElanIDs

  (as well as the reverse) in all qsw code.

 - move elanhosts.[ch] to common
 - initialize elanhost_config on demand in qsw.c
 - remove calls to elanhosts in slurmd/elan_interconnect.c
 - merge libhostlist into libcommon since elanhosts needs it.