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

[nalind]

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.

[greghudson]

I'm not going to try to apply all of krb5's coding standards to ftpd because the current ftpd style is so far off, but there are some generic quality issues with this change:

• getinput() has a complicated and undocumented contract. It would probably be less failure-prone to avoid trying to reuse the allocated buffer, and instead just have an output argument (or return value) for the allocated line which it read. In any event, the contract needs to be documented. I believe getinput() is used incorrectly by another() in ftp_cmds.c (tmp_len is not initialized by the caller), as evidence that the current contract is dangerous.
• "tmp" is a bad variable name, especially when the variable is used for multiple purposes as in getinput(). The patch should use better variable names and should not reuse variables.