[cfg] More user-friendly configuration parsing error messages

[vankoven]

Patch set #707 fixed issue #65, but TODO suggestions are not implemented.

1. Don't write configuration parsing errors directly to log, write into error string buffer inside of TfwCfgEntry instead. Display them in print_parse_error. In current implementation user gets 1-3 messages explaining why string from configuration file contain errors followed by message showing error line number and line itself. Also more user friendly messages are preferred, possibly with valid examples.

E.g. transform current output:

[tempesta] ERROR: invalid number of values; expected: 1, got: 0
[tempesta] ERROR: Unable to parse 'listen' value: 'No value specified'
[tempesta] ERROR: configuration parsing error:
                  5: listen; # 80;
                     ^^^^^^^^^^^^^
[tempesta] ERROR: failed to start modules

Into this:

[tempesta] ERROR: configuration parsing error:
                  5: listen; # 80;
                     ^^^^^^^^^^^^^
               Required argument is not set: port number or IP address and port pair.
               Examples:
                   listen 80;
                   listen 443 proto=https;
                   listen [::0]:80;
                   listen 127.0.0.1:8001;
                   listen [::1]:8001;
[tempesta] ERROR: failed to start modules

2. Indicate the token caused parsing error, e.g:

[tempesta] ERROR: configuration parsing error:
                  5: listen aa;
                            ^^
               Invalid value: expected port number or IP address and port pair.
               Examples:
                   listen 80;
                   listen 443 proto=https;
                   listen [::0]:80;
                   listen 127.0.0.1:8001;
                   listen [::1]:8001;
[tempesta] ERROR: failed to start modules

[krizhanovsky]

We need to fully rework current configuration handling (see #67 and #862 ), so there is no sense to fix the small problems.