Fix multiple Coverity warnings in tftpd(8)

[asomers]

Summary

Fix multiple Coverity warnings in tftpd(8)

• Initialize uninitialized variable (CID 1006502)
• Delete dead code (CID 1006796, 1006794, 1006793)
• strcpy => strlcpy (CID 1006792, 1006791, 1006790)
• Check function return values (CID 1009442, 1009441, 1009440)

CID: 1006502, 1006796, 1006794, 1006793, 1006792, 1006791, 1006790,
CID: 1009442, 1009441, 1009440

[asomers]

Actually, it turns out that the "dead code" wasn't dead after all; it's used by some files in the usr.bin/tftp directory. I need to restore those functions and fix their warnings the hard way.