This patch checks if the relevant functionality is actually available
before using it for authentication:
1. check if function imap_open() exists before using it
2. check if smbclient executable exists during smb auth
3. check if ftp/ftps stream wrappers are registered before
using them for auth
This fixes a crash (white page) that happens when using IMAP authentication
and the IMAP extension is not installed.
cd05354aaf