spurious "Use of "-x" without parentheses" mesg

RT #120288.

Something like '-X' at the start of a line (where X is one char, but not a
filetest operator) was giving a spurious

    Warning: Use of "-a" without parentheses is ambiguous

error. This was because the toker was provisionally marking the -a as the
last encountered unary operator (PL_last_uni), then not unmarking it when
it found that that it wasn't in fact a filetest operator.