Autofill credentials in the SAML window from ini file

[nitrogl]

This allows one to autofill credentials in the SAML window from ini file.

The ini file is "~/.gpclient-credentials"; the gateway/portal must match the section in the ini file (e.g., [vpn.domain.com]).

Limitation: the SAML login page must have "username" and "password" fields to be filled correctly. The fields are hardcoded, due to the inih inability to surf through sections and fields. Please, consider moving to other c++ ini readers, e.g., https://github.com/nitrogl/cinir [disclaimer: I developed it].

Mode of the credential file should be set the to 600 (similarly to .git-credentials, .ssh/id_rsa, ...).

[nitrogl]

This should also "fix" issue #185