Use global zero timeout

Global zero timeout is used to prevent ldap_result. Since the
ldap_result is being run only when some data are ready in buffer, it is
not necessary to wait for any time period.
Specify search attributes in IMAP module

Search attributes must be specified in IMAP module because it varies
according to operation being executed. For simple login we need to look
for 'mailHost' attribute only. But 'authenticate' operation needs Skeeter
to validate client's credentials before authenticating with admin account
on user's behalf. In such cases a 'password' attribute is requested.
Style changes and typo correction

Correct error message
Cosmetic changes in ldap_connect_cb
Remove useless function declarations
Add msg freeing into ldap_read_cb
Purify ldap_bind_cb
Replace malloc with calloc