Don't limit the length of an input line in FTP

This set of chages replaces the FTP client's use of static-sized "line" and "argbuf" buffers and fgets() to process command input with dynamic buffers and a knockoff of getline() that we'll call getinput().

It leaves the limit on the number of arguments unchanged, and makes a guess that macro expansion won't require more than 500 chars beyond the length of the macro definition and the current command, as it was entered.