MFC r336605:

Fix multiple Coverity warnings in tftpd(8)

• Initialize uninitialized variable (CID 1006502)
• strcpy => strlcpy (CID 1006792, 1006791, 1006790)
• Check function return values (CID 1009442, 1009441, 1009440)
• Delete dead code in receive_packet (not reported by Coverity)
• Remove redundant alarm(3) in receive_packet (not reported by Coverity)

Reported by: Coverity
CID: 1006502, 1006792, 1006791, 1006790, 1009442, 1009441, 1009440
Differential Revision: https://reviews.freebsd.org/D11287