Added xerrno.[c,h] and used it in qsw.c and runqsw.c.

Changed a couple of names in the qsw.c api:
 qsw_prog_reap -> qsw_prgdestroy (since it's just a wrapper for rms_prgdestroy)
 qsw_attach -> qsw_setcap (since it's just a wrapper for rms_setcap)
 qsw_signal_job -> qsw_prgsignal (since it's just a wrapper for rms_prgsignal)
Added a conditional include of qsw.h to slurmctld.h so it would compile
when configured --with-elan.