user_external: Check if extensions are enabled before using them

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.
Merge pull request #1614 from cweiske/external-check

user_external: Check if extensions are enabled before using them