Fix error reporting of templates

[MrAnno]

log_template_compile() has a return value, which can indicate compilation error.

Unfortunately, the return value was not checked in several places, producing repeated run-time errors and unexpected behavior.

For example:

options {
	create-dirs(yes);
};

log {
	source { stdin(); };
	destination {
		file("/tmp/${DATE");
	};

	flags(flow-control);
};

The above config snippet created a file named ${DATE inside the following directory structure:

$ tree .

error in template: /
└── tmp
    └── ${DATE

This is because the log_template_format() method called on an invalid template will contain the error itself (error in template: ...).

This PR adds proper error handling where templates are used, or where possible, just replaces the template creation logic with the template_content grammar rule, which takes care of error reporting:

Error parsing affile, Error compiling template, error=Invalid macro, '}' is missing, error_pos='11' in /home/anno/dev/syslog-ng/install/etc/syslog-ng.conf:39:8-39:21:
34      };
35
36      log {
37              source { stdin(); };
38              destination {
39---->                 file("/tmp/${DATE");
39---->                      ^^^^^^^^^^^^^

The PR contains some additional unit tests for non-trivial templates, and also makes empty templates (template("")) trivial and adds log_template_is_literal_string(), so we can get rid of the strchr($) trickery in the file destination.

[kira-syslogng]

Build SUCCESS

[MrAnno]

@kira-syslogng do stresstest

[kira-syslogng]

Build SUCCESS

[kira-syslogng]

Kira-stress-test: Build SUCCESS

[Kokan]

Thanks.

[kira-syslogng]

Build SUCCESS

[gaborznagy]

Besides a minor thing, approve. 👍

[kira-syslogng]

Build SUCCESS