Add unistd.h to c.h

Twenty percent of our C files include unistd.h.  Should we include
unistd.h in c.h and remove mentions of unistd.h in files that include
c.h?

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +