ftpd: fix command fetching to not do it in 1-byte reads;

ftpd: fix command fetching to not do it in 1-byte reads;
 fix command de-escaping. Tested to download files with embeeded \xff and LF.
libbb: tweaks for the above

function                                             old     new   delta
ftpd_main                                           2231    2321     +90
xmalloc_fgets_internal                               190     222     +32
xmalloc_fgets_str_len                                  -      27     +27
xmalloc_fgets_str                                      7      23     +16
xmalloc_fgetline_str                                  10      26     +16
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(add/remove: 1/0 grow/shrink: 4/0 up/down: 181/0)             Total: 181 bytes