Show sign-only key algorithms by default

[Yegor Ievlev]

By default "ssh -Q key" doesn't show more secure rsa-sha2-512 and
rsa-sha2-256 key algorithms. This PR shows them unless the user
specifies that they only want key algorithms that support encryption.

ssh.c:
744c744
< cp = sshkey_alg_list(0, 0, 0, '\n');
---
> cp = sshkey_alg_list(0, 0, 1, '\n');
746c746
< cp = sshkey_alg_list(1, 0, 0, '\n');
---
> cp = sshkey_alg_list(1, 0, 1, '\n');
748c748,750
< cp = sshkey_alg_list(0, 1, 0, '\n');
---
> cp = sshkey_alg_list(0, 1, 1, '\n');
> else if (strcmp(optarg, "key-encrypt") == 0)
> cp = sshkey_alg_list(0, 0, 0, '\n');

Please set email of commit to koops1997+github at gmail.com and name to
Yegor Ievlev, so commit will be attributed to me on GitHub.