include system headers required for got_date.h directly, not indirectly

got_date.h: add explicit #includes Don't rely on implicit/"hidden" includes, but rather add them to the header file so their inclusion doesn't result in missing values. Although not explicitly a functional change, this does help -portable. This ensures size_t/time_t are always present. OK @jrick

create and verify tags signed by SSH keys This adds a new -s flag to 'got tag' that specifies the signer identity (for example, a key file) of the tagger. The tag object will include a signature that validates each of the tag object headers and the tag message. Verifying these signed tags requires maintaining an allowed signers file which maps signer identities (i.e. the email address of the tagger) to SSH public keys. See ssh-keygen(1) for more details of the allowed signers file. After creating this file and providing the path to it in got.conf(5) using the allowed_signers option, tags may be verified using with 'got tag -V tag_name'. The return code will be non-zero if a signature fails to verify. ok stsp@