Use system sslcacertfile by default

[thekix]

This patch includes a new functionality to get the sslcacertfile.

With this patch is possible to remove the sslcacertfile variable form the
.offlineimaprc and use the system's default. This could be easy for new users.

With this patch, the user can use ssl without the sslcacertfile option
in .offlineimaprc, use sslcacertfile = (no include anything
after the equal sign, os specify the sslcacertfile as previosly.

Currently, these are the options:

User specifies sslacertfile -> OK
User specifies sslacertfile=OS-DEFAULT
    If the system's default is provided -> OK
    If the system's default is not provided -> Fail
User do not specifies sslacertfile -> Fail
User specifies empty sslacertfile -> Fail

With the new option (this patch).

User specifies sslacertfile -> OK
User specifies sslacertfile=OS-DEFAULT or
User do not specifies sslacertfile or
User specifies empty sslacertfile
    If the system's default is provided -> OK
    If the system's default is not provided -> Fail

Closes #14

This v1.1 template stands in .github/.

This PR

Add character x [x].

• I've read the DCO.
• I've read the Coding Guidelines
• The relevant informations about the changes stands in the commit message, not here in the message of the pull request.
• Code changes follow the style of the files they change.
• Code is tested (provide details).

References

• Issue #no_space

Additional information