MFC r343165 (by jhibbits):

Fix top(1) long options handling

getopt_long(3) requires the long options be terminated by a NULL block.
Without the terminator, an invalid long option results in a segmentation
fault.

Reported by: Brandon Bergren

MFC r343957:

Fix multiple warnings in usr.bin/top about discarded qualifiers from
both clang and gcc, by either constifying variables, or when that is not
possible, using __DECONST.

MFC r343958:

Fix multiple warnings in usr.bin/top about variables shadowing global
declarations from base gcc, by renaming those variables.

MFC r343959:

Fix the first couple of AddressSanitizer violations in usr.bin/top.

Avoid setting zero bytes beyond the length of the 'thisline' parameters
in i_process() and u_process(), and don't attempt to memset a negative
number of bytes.