Teach dnssec-settime to read times that it writes

The dnssec-settime -p and -up options print times in asctime() and UNIX time_t formats, respectively. The asctime() format can also be found inside K*.key public key files. Key files also contain times in the YYYYMMDDHHMMSS format that can be used in timing parameter options.

The dnssec-settime -p and -up time formats are now acceptable in timing parameter options to dnssec-settime and dnssec-keygen, so it is no longer necessary to parse key files to retrieve times that are acceptable in timing parameter options.

Backport of !2947 (merged)