performed by the pam_conv (PAM conversation) callback, which then
getline()s the PASS reply internally. This involves calling
auth_pam() from user() and caching the result to use later in pass().
This allows the PAM modules to present a different password prompt
dialog if necesary. For example:
Name (localhost:lukem):
331 User lukem accepted, provide password [ otp-md4 89 xxxx12345 ].
versus
Name (localhost:lukem): root
331 User root accepted, provide password.
This is independent of (and effectively exclusive to) USE_SKEY support.
Previously ftpd with USE_SKEY=yes would provide the skey prompt
if the user had an skey configured, even if /etc/pam.d/ftpd didn't
have pam_skey in use.
I.e., ftpd shouldn't need special support for custom password prompts
(such as skey) if PAM is in use.
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