Large SOA serial numbers are not handled properly by "signer"

If the SOA serial number of the input unsigned zone is larger than 2^31-1 (0x7fffffff or 2147483647), the generated signed zone always has a serial number of 2^31-1 if "keep" or "counter" is used.

The problem seems to arise due to the use of the atol function in signer/tools/signer.c (handle_command function). Since atol converts to a signed long, its valid range limit is (-)2^31 to 2^31-1. A param value greater than 2^31
- 1 causes it to return LONG_MAX or 2^31-1.

A possible fix is to use strtoul instead of atol.

Although this problem is seen for SOA, in theory it could occur for all the uint32_t parameters. The attached signer.c uses strtoul instead of atol for all uint32_t variables although this may not be strictly necessary e.g. TTL should never exceed 2^31-1.

A sample unsigned input zone with a large serial number and the corresponding signed zone with incorrect serial is also attached.