MFC r336609:

Fix several Coverity warnings in tftp

Some of the changes are in the libexec/tftpd directory, but to functions that
are only used by tftp(1) (they share some code).

• strcpy => strlcpy (1006793, 1006794, 1006796, 1006741)
• Unchecked return value and TOCTTOU (1009314)
• NULL pointer dereference (1018035, 1018036)

Reported by: Coverity
CID: 1006793, 1006794, 1006796, 1006741, 1009314, 1018035
CID: 1018036