CVE-2026-44656: fix shell command execution via backticks in 'path' option
during file completion by skipping backtick entries in expand_path_option()
(findfile.c, upstream patches 9.2.0435 and 9.2.0444; the P_SECURE-on-'path'
portion of 9.2.0435 was reverted by 9.2.0444 and is intentionally not
backported)