also apply generator limit to use with CIDR

also apply generator limit to use with CIDR

As described in GH issue #299, the MAX_GENERATE+1 limit is only
applied when using an address range, not when using CIDR.  This
commit changes this to always honor the generator limit.

* refactor target address generation to use the same new function
  for both range and CIDR notation
* check the limit for addresses to generate in the new function
* document the generator limit in "fping --help" output
* document the generator limit in the fping man page
* test that the address generation limit applies when using CIDR
  notation