ftpd: new option -a ANON_USER to allow anonymous logins

ftpd: new option -a ANON_USER to allow anonymous logins

Anonymous ftpd login is useful even when ftpd authentication feature
is enabled. Anonymous logins provide simple password-less connection
for FTP clients.

To allow password-less connection user command line option '-a USER' is
added. This option specifies the system user to use when
'anonymous' username is given in USER command. No password is required
in this case.

function                                             old     new   delta
ftpd_main                                           2164    2232     +68
packed_usage                                       31015   31046     +31
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(add/remove: 0/0 grow/shrink: 2/0 up/down: 99/0)               Total: 99 bytes

Signed-off-by: Andrey Mozzhuhin <amozzhuhin@yandex.ru>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>