FTP support

[semyon-san]

What do you think of adding FTP support using curlftpfs? One could create a copy of sftpman (name it ftpman) and just substitute sshfs commands with curlftpfs but that's just silly.

One can solve it it two ways:

1. Add protocol option to config file of sftpman to specify if you want to mount using ftp or sftp (easier way)
2. Extract code base from sftpman that manages the mounted servers and name it "mounting engine". Then different implementations for different protocols (sftpman, ftpman) use the mounting engine but specifiy their unique commands to mount/unmount and parsing of their config files. (harder but better)

[spantaleev]

Sounds somewhat useful, but also like something which would introduce complexity on multiple levels:

• the required dependency libraries/programs are different
• the configuration options (authentication, etc.) are different depending on protocol

This would affect: packaging, internal management code, CLI interface, and even the GTK interface (sftpman-gtk).

Additionally, #4 should probably be implemented, as mounting under /mnt/sshfs using FTP doesn't make sense. Well, that, or we'd need to stop using /mnt/sshfs in favor of something better, which fits both SFTP and FTP.