BUG/MINOR: mworker: section ignored in discovery after a post_section_parser

BUG/MINOR: mworker: section ignored in discovery after a post_section_parser

When a new section is discovered, the post_section_parser of the
previous section is called. However in the new master-worker mode the
discovery mode will skip the post_section_parser. But instead of
trying to parse the current section keyword after that, it would skip
completely the current line.

This is a minor bug since there isn't a lot of section with
post_section_parser, and not a lot of section to parse in discovery
mode.

But this could be reproduced like this:

	global
	        expose-deprecated-directives

	resolvers res
		parse-resolv-conf

	program foo
	        command sleep 10

	program bar
	       command sleep 10

Ths 'resolvers' section has a post_section_parser which will be ignored
in discovery mode with the consequence of ignoring the first program
section.

This must be backported in 3.1.

(cherry picked from commit 2c270a05f06ff6ea2c23b369692a12a7780291b7)
Signed-off-by: Willy Tarreau <w@1wt.eu>