Several compilation and run-time problems occur when building on SGI

IRIX systems using the native compilers.  A summary is:
- Various files use "//" as a comment delimiter in c files.
- Problems caused by assuming "char" is signed.
  cash.in: building -signed the rules regression test fails as described
    in FAQ_QNX4.  If CHAR_MAX is "255U" then ((signed char)CHAR_MAX) is -1.
  postmaster.c: random number regression test failed without this change.
- Some generic build issues and warning message cleanup.

David Kaelbling