src/login.c: Fix off-by-one buggs

src/login.c: Fix off-by-one buggs

Before 3b7cc053872c ("lib: replace `USER_NAME_MAX_LENGTH` macro"), this
code did use a length.  It used a utmp(5) fixed-width buffer, so the
length matches the buffer size (there was no terminating NUL byte).
However, sysconf(_SC_LOGIN_NAME_MAX) returns a buffer size that accounts
for the terminating null byte; see sysconf(3).  Thus, the commit that
introduced the call to sysconf(3), should have taken that detail into
account.

403a2e3771be ("lib/chkname.c: Take NUL byte into account"), by Tobias,
caught that bug in <lib/chkname.c>, but missed that the same commit that
introduced that bug, introduced the same bug in two other places.
This fixes all remaining calls to sysconf(_SC_LOGIN_NAME_MAX).

I still observe some suspicious code after this fix:

	if (do_rlogin(hostname, username, max_size - 1, term, sizeof(term)))

	...

	login_prompt(username, max_size - 1);

We're passing size-1 to functions that want a size.  But since the fix
to those will be different, let's do that in the following commits.

Link: <https://github.com/shadow-maint/shadow/pull/935>
Link: <https://github.com/shadow-maint/shadow/issues/920#issuecomment-1926002209>
Link: <https://github.com/shadow-maint/shadow/pull/757>
Link: <https://github.com/shadow-maint/shadow/issues/674>
See-also: 403a2e3771be ("lib/chkname.c: Take NUL byte into account")
Fixes: 3b7cc053872c ("lib: replace `USER_NAME_MAX_LENGTH` macro")
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Cc: Tobias Stoeckmann <tobias@stoeckmann.org>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>