Remove extern declarations of library functions from source.c.

  This could cause problems on odd systems.  The generic
  headers should be extended as needed to include necessary
  system headers or, if necessary, make explicit declarations.
Extended ac/string.h header to look for string.h/strings.h if
  STDC_HEADERS is not defined.  Also provide basic declarations for
  str*() functions.  This could cause problems on odd systems.
Extended ac/unistd.h header to define basic declaration for misc
  functions that might be missing from headers.   This includes
  externs for getenv(), getopt(), mktemp(), tempname().
Protect fax500.h from multiple inclusion.  Moved includes of
  system/generic headers back to source files.
Made mail500 helper functions static.
Fixed includes of ctype.h, signal.h, etc. to use generics.
lutil/tempname.c: was including stdlib.h twice, one should stdio.h.
Wrapped <sys/resource.h> with HAVE_SYS_RESOURCE_H.
lber/io.c/ber_get_next(): Changed noctets back to signed.
  Used with BerRead which expects signed int as second arg and
  returns signed int.