BUG/MEIDUM: startup: return to initial cwd only after check_config_validity()

BUG/MEIDUM: startup: return to initial cwd only after check_config_validity()

In check_config_validity() we evaluate some sample fetch expressions
(log-format, server rules, etc). These expressions may use external files like
maps.

If some particular 'default-path' was set in the global section before, it's no
longer applied to resolve file pathes in check_config_validity(). parse_cfg()
at the end of config parsing switches back to the initial cwd.

This fixes the issue #2886.

This patch should be backported in all stable versions since 2.4.0, including
2.4.0.

(cherry picked from commit e900ef987e4167cf0921e97b09059d757f2c0ea5)
Signed-off-by: Willy Tarreau <w@1wt.eu>